I would NOT rely on GTA 5 PC being able to be played in OFFline Story mode. After installing some mods I tried playing offline, and found I could usually get it to work the first time I start Steam in offline mode. But after a game crash or just closing the game and going back to it later, it always gave me a 'Social Club login' error. Maybe a PC restart would fix it, or maybe there's a 'time out' between offline tries.