Modding for GTA was always a trump card of the PC versions.
The modding scene is solely based around the PC version of GTA. There are a few ways to get or convert existing PC mods for console use on previous games like Vice City or San Andreas, but the work involved into making sure everything runs fine is staggering.
I think people have been trying to copy the game onto the HDD and modify it from there, not sure it they managed any headway but it's probably a lot easier these days. Better off asking someone who is active on the modding scene