Honestly, I wish they just made the multiplayer an improved version of Red Dead Redemption's, which was an improved version of GTA IV's. Online is almost like it in a way, except it has this really loose and vague story to it, and everything costs money. At least in RDR all you had to do was level up to get a new weapon or horse, or if you found a weapon in freeroam you would keep it and not have to level up for it.
Aside from that, currently there are no anti-cheat measures. The only thing right now is to report a modder and that's it. There's no file check or anything, people can just open their game's files, edit some values, then have millions of dollars and have the ability to give it to people. Though, Rockstar has said they're working on something - but it's practically too late now.