It totally depends on the situation. If it was a gorilla fighting only one hyena the gorilla would likely win hands down. However, I think an entire pack of hyenas would have the intelligence to completely surround the gorilla, attacking its sides and hindquarters with the super strong bite it supposedly has. Although the gorilla is the largest of all primates and has upper body strength around six times that of an adult human, I think it'd eventually succumb to the pack.