Francia, the European super-power of the time, referred to Borna as dux Guduscanorum, dux Dalmatiae atque Liburniae – the Duke of the people of Gacka, Dalmatia and Liburnia. It is considered that the area which Gacka covered back in the day was much wider than the one at the present, stretching from the Raša River in Istria all the way to the Una River in the east. In a later period, Gacka gradually lost its territories thus narrowing down in size to the present time area. In the mid-10th century, the Byzantine Emperor, writer and philosopher Constantine VII Porphyrogenitus mentions the Croatian counties. Three of them, among the others, stood out as different. Those counties were Gacka, Lika and Krbava. What made them different was the fact that they were ruled by a Ban, while the other counties were ruled by a Duke. In the early 10th century the province of Gacka belonged to the first historically known, Banovina of Croatia.

The area of Gacka had been inhabited as early as the prehistoric times. The oldest site from this period in the Gacka Valley is located on its southern edge within the Pećina in Lešće. The traces of charred bones of wild animals as well as bones showing damage made by the usage of stone tools reveal the presence of humans from the Mesolithic period - the transition from early to late Stone Age (10000-4000 BC). Back then, man still lived the lifestyle of a hunter-gatherer.

Somewhat earlier habitats and traces of human settlements are found precisely within the area of the Brinje municipality. The cave of Siničić špilja, on the way from Brinje to Letinac, has only recently unveiled its secrets to the scientific community. Inside the cave there lived a Paleolithic hunter in (9500 BC) from the late Mesolithic period, who left the, still undeciphered, leptolithic engraved pictorial representations on the surface of the smooth rocks within the cave. A group of notches similar to Germanic runes was probably a primitive lunar calendar of sort. Remains of pottery were also found within the cave. Man of the time was still a hunter-gatherer getting used to a new, sedentary way of life.

Traces from the middle and late Bronze Age were found in the area of Vrhovine. In the cave of Bezdanjača under Vatinovac hill, just two kilometers west of the Vrhovine toward Zalužnica, a necropolis from 1400 BC was discovered. The cave consists of two parts: in Prehistoric times, the first, vertical part below the pit was used as a dwelling (later, the main settlement moved to the Vatinovac hill), while the other sloped greater part was being used as a necropolis.
With more than 250 skeletons discovered, it is one of the largest cave necropolises in Europe. Man has already adopted the new way of life, breeding and raising domesticated animals (cattle, sheep, goats, dogs) and cultivating grains. It is evident from the remains of the material culture that the group of people living in the region belonged to the Proto-Illyrians, predecessors of later Illyrian tribes. In the late Bronze Age in the 9th and 8th centuries BC, the Illyrian tribe Iapydes occupied the entire area of Gacka and beyond. Iapydes culture was very rich and vivid, and partly differs from cultures of other Illyrian tribes.

It was exactly the sites from withing the Gacka area where the most valuable archaeological findings of these tribes were excavated. Iapydes mostly preffered to live inside of forts which were built on top of low rounded hills: in Crkvina in Kompolje (Avendo), in Veliko and Malo Vitlo in Prozor (Arupium), in Veliki and Mali Obljaj east of Vrhovine. It is widely considered that the municipality of Brinje was named Monetium back in the day. Other such forts were discovered in Drenov Klanac, Staro Selo and Švica (as many as 23 ruins had been discovered within the area of the Gacka Valley alone).
The culture of copper processing and production of exceptionally rich jewelry (especially the jewelry made of amber and glass) has experienced a renaissance in the early Iron Age (8-5th centuries BC). In these rich findings, what truly stood out were the beautifully processed Iapydic headgear and hats. The ethnologists consider these items to be the precursor of today's Dinaric type, short fringed male red cap (some may call it Lika or Gacka cap). Their later successors have not only taken the Iapydic cap, but other parts of the traditional male and female costumes as well. The Iapydes from the modern day Gacka area had shared cultural ties with Etruscans, Greeks and, across Pannonia, cultures of the Black Sea as well.

During the 2nd century BC, the Romans began to disturb the peaceful life of the Iapydes. The Iapydes had been at war with the Romans for nearly two centuries, essentially playing tug of war with the ever changing mixed results on both sides. They were finally conquered by the Roman Emperor Octavian in 35 BC.
With the arrival of the Romans, the present area of Gacka finally received the ever needed traffic importance. Along with the construction of the road from northern Italy to Greece, the Roman authorities, gradually brought changes to the lives of the native population. They built upon, expanded and fortified Arupium in Veliko and Malo Vitlo in the present day Prozor as their central settlement, from which they had the perfect grasp of the region.

According to archaeological remains, it was an important site with public buildings, pagan temples and a high standard of living for the time (primitive floor heating). Arupium was a central Roman settlement in the areas of Gacka, Lika and Krbava from which they controlled the main road and the intersection to Dalmatia and Pannonia.
The lively traffic activity brought a multitude of Roman soldiers to Gacka, together with their religions and beliefs. In the 2nd and 3rd centuries a widely spread cult of Mithras, the Sun-God, was brought from Senia (the present day Senj). In terms of artistic treatment, the Mythrian shrines in Čović and Sinac are of Italic origin, while the shrine in Špilničko polje east of Otočac is of Noric-Pannonian origin.

With the weakening grasp of the Roman Empire, the barbarian people of the eastern and northern Europe started to invade the Valley of Gacka. At the end of the 5th century, Goths arrived and established their short-lived authority in the area of Dalmatian Croatia. According to some sources, the name of the province of Gacka is derived from the name of the Germanic tribe of Goths, meaning 'small state of the Goths'. After the Goths, who partly remained living in this area and mixed with the natives, in the mid-6th century came the Slavs. The Avars invaded the area at the end of the 6th century, and the Croats came in the 7th century. According to historical sources, it appears that in this difficult area in terms of climate and relief these different nations were more tolerant of each other and found a way to co-exist.
Proof of their coexistance lies in the fact that in the mid 10th century, Constantine VII Porphyrogenitus wrote that there were still some Avar tribes remaining within Croatia, who differed in appearance from other people inhabiting the land.

It seems that the largest number of them were found exactly within the province of Gacka.
With the arrival of Croats to the Gacka Valley and it's region, while spreading their name and influence on other nations with Francia, the European super-power of the time, the first centre cores of the Croatian medieval state were formed right here in the Valley of Gacka. Duke Borna expanded his authority further from Gacka to the regions of Dalmatia and Liburnia thus creating the first larger area state under the Croatian name. In the late 9th and early 10th centuries, Gacka and the neighboring Krbava and Lika were under the rule of a Ban (viceroy). During the time of the rule of the Croatian kings, Gacka lost on importance as the central governing area, which then shifted to the areas of northern and central Dalmatia.

When Hungarian kings assumed power in Croatia, Andrew II, King of Hungary and Croatia, gave the province of Gacka to the Knights Templar in 1219. Shortly afterwards (in 1241), the Valley of Gacka was devastated by the Mongol invasion. Batu Khan broke with his horde over the Kapela hill to catch the Hungarian-Croatian King Bela IV. The Templars did not retain Gacka in their possession for long. King Bela took Gacka from them just to trade the region for another possession of property back in 1269.
The late 13th century saw the emergence of the counts of Krk, the later Frankopans (Frangepanes). Gacka became their property in 1290, remaining in their possession until as late as 1468 when it was taken over by the Hungarian-Croatian King Matthias Corvinus.
The Frankopans had the following fortified towns in Gacka: Otočac, Prozor, Vrhovine, Dabar, Brlog, Sokol, and Jelovik.

Apart from fortifying and establishing towns and forts, they also built churches, donated monasteries, and helped develop the culture and literacy in general. Their main centers were located in Otočac and Brinje where their rule left the biggest influence on the local population and sparked the growth of culture within the area. During the reign of Sigismund of Frankopan, in 1460, the diocese of Otočac was established and lasted up until the Ottoman invasion of 1534.
In the late 15th and 16th century, during the 100 year War with the Ottomans, the Turks invasions were becoming more and more frequent, violent, and dangerous. Both Lika and Krbava regions fell under the Ottoman rule back in 1527. As a result of those circumstances, Gacka and Otočac both became border areas exposed to frequent Ottoman attacks. In the early 16th century, Gacka, especially the Valley of Gacka together with the area of Vrhovine had experienced the exodus of the local population, which was slightly less felt in the Brinje area, due to it's location. It was the time of the population's massive emigration and flight to the Adriatic littoral, Gorski Kotar and Carniola, as well as to northern Croatia, Hungary and Austria. The only populated areas which remained were the town of Otočac, and the municipalities of Brinje and Prozor.

To defend from the invading Ottomans, Hungarian King Matthias Corvinus established the military captaincy of Senj in 1469. The following fortifications belonged to the captaincy in 1538: Otočac, Prozor, Brlog, Brinje, Senj, Jablanac, and Bag. In 1578, the military defense was reorganized and the Primorska krajina established. Otočac and Brinje would remain the two centennial strongholds for the defense of Gacka. Until the final expulsion of the Ottomans from Krbava and Lika in 1689, several battles were led and won against them from within the Gacka Valley. The Count Petar Zrinski defeated the Turks with help of the people of Otočac twice in 1655. In 1657, the soldiers from Otočac united with the soldiers of Senj to defeat the common enemy, an army of 6000 strong. The Turks were defeated in the Gusić Polje. In 1663, with an army of 2000 soldiers, Petar Zrinski and Fran Krsto Frankopan defeated 8000 Turks near the locality of Jurjeve stijene. Thus they finally prevented further Turkish incursions into the inner, free Croatian territory. Immediately after the expulsion of the Turks from Krbava and Lika, where Marko Mesić, a Serbian orthodox priest from Brinj excelled in particular, the Military Frontier suffered a new reorganization. The entire area of Gacka was subjected to the authority of the Military Frontier Administration in Karlovac. With the disappearance of the Turkish threat in the late 17th century, immigrants from Gorski Kotar, Carniola and northern Dalmatian coast systematically settled in the deserted area of the Gacka Valley. In 1746, the glorious regiment of Otočac was formed from the old military captaincy of Otočac. It comprised the area from the coast all the way to the east to the Bosnian border. It would last as late as until the demobilization of the Military Frontier in the 19th century. However, the repressive system of the Military Frontier, the largest barracks in whole of Europe, had caused discontent and a start of the rebellion of the people of Brinje in 1746. The rebellion was crushed and the rebels brutally punished by the authorities.

From the 18th until the mid-19th century, the people of Gacka took part not only in the wars with the Ottomans, but also had  defended the Austrian Empire as well, while fighting for the Viennese court all across Europe. In these wars, especially in the suppression of the Hungarian revolution in 1848, they impressed with their bravery, and as such were awarded respectable imperial decorations. After the demobilization of the Military Frontier in 1881, the region of Gacka joined the civilian Croatian state and lived on to see the dissolution of the Austro-Hungarian Empire in 1918.The entire area of Gacka was subjected to the authority of the Military Frontier Administration in Karlovac. With the disappearance of the Turkish threat in the late 17th century, immigrants from Gorski Kotar, Carniola and northern Dalmatian coast systematically settled in the deserted area of the Gacka Valley. In 1746, the glorious regiment of Otočac was formed from the old military captaincy of Otočac. It comprised the area from the coast all the way to the east to the Bosnian border. It would last as late as until the demobilization of the Military Frontier in the 19th century. However, the repressive system of the Military Frontier, the largest barracks in whole of Europe, had caused discontent and a start of the rebellion of the people of Brinje in 1746. The rebellion was crushed and the rebels brutally punished by the authorities. From the 18th until the mid-19th century, the people of Gacka took part not only in the wars with the Ottomans, but also had  defended the Austrian Empire as well, while fighting for the Viennese court all across Europe. In these wars, especially in the suppression of the Hungarian revolution in 1848, they impressed with their bravery, and as such were awarded respectable imperial decorations. After the demobilization of the Military Frontier in 1881, the region of Gacka joined the civilian Croatian state and lived on to see the dissolution of the Austro-Hungarian Empire in 1918.

 With the dissolution of the Austro-Hungarian Empire, Gacka took part in the southern Slavic community for the first time. Since 1939, it became a part of the Croatian Banovina. In 1941, Otočac and Vrhovine were attached to the Great County of Gacka and Lika while Brinje was a part of the Great County of Vinodol and Podgorje. In World War II, the founding session of ZAVNOH, the National Anti-Fascist Council of the People's Liberation of Croatia, was held for the first time in Otočac. At the end of World War II, Otočac and Brinje became county towns and Vrhovine a municipal town.

In 1991, the people of Gacka experienced a catharsis in the war against Serbia and the local Serbian rebels. One third of the territory of Gacka in the northeast was occupied. During the violent attacks, Brinje and Otočac, and the surrounding villages in particular, suffered major destruction and many casualties. In the new territorial and administrative organization of Croatia, Otočac received the city status in 1993, while Brinje and Vrhovine once again became municipalities. The city of Otočac and the municipalities of Brinje and Vrhovine became a part of Lika-Senj County.