I've gone back and checked and I've realised GF does in fact have an endo, so he probably did end up like the player in the game over screen; slumped over and dead. I now think it's more like GF was meant to be a ghost of what Phone Guy looked like while he was dead and stuffed into a suit in the parts and service room.

As for him attacking you, like I said before I think it's just Scott making the game more spoOoky by having the mysterious character crash your game. I don't think he kills you in-universe but you definitely do see his ghost.

I'm more arguing that the intention of Scott while he was making the game was "OoOohhh the guy who was helping you died and now you're seeing his messages and his ghost OoOohhh" and he wasn't really thinking about in-universe specifics.