Gorilla City was a high-tech civilization whose residents were gorillas with intellects at the level of (or possibly superior to) those of mankind.
In the 1500s, Gorilla City was originally a Kree outpost in Africa where Kree scientists experimented Earth's gorillas and giving them higher intelligence. After the Kree were repelled from Earth, the intelligent gorillas took over the outpost and renaming it as Gorilla City and remained hidden from the outside world for centuries.
The existence of the city was eventually revealed to humans when Grodd, the gorilla responsible for the city's security, made contact with Erik Killmonger and offering to assist him in his attempted coup of Wakanda in exchange for Killmonger's help in overthrowing Solovar. However, while tracking Killmonger's forces, Gorilla City was discovered by heroes Superman, Flash, Black Panther, Storm, Kurox, Cyclops, Steel, Phoenix, Green Arrow, Phantom, Android, and Spider-Man, with Solovar accepting their word that they would keep the city's existence secret due to the noble reputation of the Panther Clan in Africa. Although Chloe Sullivan was briefly tempted by the story potential of Gorilla City's existence after she followed the heroes, she agreed to maintain its secrecy after she recognized that some of the gorillas were only children who didn't deserve to be exposed to the public.
When the Kree and Skrulls attempted to invade Earth, Earth's remaining heroes put Gorilla City in touch with the Inhumans, reasoning that the two sides' mutual experience with Kree technology could be useful.
In DC Comics, Gorilla City was formed by the hyper-intelligent gorillas that were affected by a crashed alien spaceship. The Flash first finds out about the city due to Gorilla Grodd, who probed Solovar's mind to find how to control minds, before trying to take over Gorilla City, then the world. However Flash defeated him. During JLA Annual #3 (the JLApes storyline), Gorilla City reveals itself to the world and joins the United Nations. However Solovar was killed by a bomb apparently set off by human terrorists.