If you are like me, Steam is the first place to check for new released games. And the reason that Grand Theft Auto V is not on Steam right now is because the game was only released for consoles, Xbox and PlayStation. Not because Rockstar has a similar platform as EA (Origin).
All the other Rockstar games are available on Steam, GTA 3, Vice City, San Andreas and IV.
GTA V will be released at some point for PC and the version will definitely be available on Steam. The Internet rumors are saying something about five-six months.