I'm willing to argue that he's shit at GTA V too.
His issue is relevant to all games, and that is his interface blindness. He is very unwilling to listen to dialogue, read subtitles, or read text elements on the screen.
He doesn't understand why things happen in cutscenes because he doesn't listen to the dialogue or read the subtitles. He keeps asking where the boost button is because he doesn't read the interface and doesn't bother to memorize vehicles. The only "good" he's at in GTAV is because of literal thousands of hours of repetitive training and practice. Outside of this extremely narrow comfort zone, he's got the adaptability and talent of a helpless toddler.
Like, I figured he's merely pretending to be retarded, repeatedly and constantly displaying such behavior in order to drive engagement in comments and from the chat. But I recently checked out a vod of him playing GTA San Andreas for the first time, with only a fraction of the notoriety and audience he has now, and he displays all of the issues that I listed. No idea where to go, no idea what to do, no idea why the thing just happened, no idea why he needs to go there. It's infuriating to watch and I'm convinced it's not an act, but genuine retardation.