Located in Ganton, Los Santos, the gym becomes accessible following the mission Drive-Thru. Carl Johnson can learn boxing moves at the gym, as well as use the other facilities to increase or decrease certain statistics, specifically stamina, muscle and fat.
One common use for the gym is the hole in the roof which allows access to Blue Hell, which requires the player kill everyone inside, using the jetpack cheat code to acquire a jetpack, and walking towards the door with the jetpack, lifting the player off into Blue Hell. Using this the player can enter several non-used interiors, you can even enter Liberty City, which is a left over object from the mission Saint Mark's Bistro.
Another glitch with the gym is to park a vehicle (trucks and bikes usually don't work though) in front of the entrance to the gym, and then enter it. After that, going back outside, the door will be blocked by the vehicle and the player pushed back into a hallway. Only the floor of the hallway is solid, and going outside of the room will make the player fall and reappear in the street.