I've come back to this problem myself several times and I've tried using some programs that can map the buttons of my gamepad to keys.
But with these I've always run into the same problem of getting into a vehicle and the different control scheme messing everything up. Has anyone else come up with a solution?
Best Answer
Bit late, but try GInput: https://gtagmodding.com/mods/107-ginput/
It integrates XInput, which is used for a majority of games supporting Xbox360 controllers. However, XInput can also be patched with an emulator for other types of controller.