(In my humble opinion) I think the key difference between response to a game and response to reality is the fact that the player knows one is make-believe before they engage in it. This fore-knowledge that you're in a simulation coupled with a desire to be there goes a long way to blunt the response your mind has to the input.
If it's possible for games to cause PTSD, then I imagine it would be possible to get PTSD from film and books as well. I'm not suggesting that this isn't the case however; But I do think it likely that people who are prone to being overly stressed by experiencing traumatic events would be more likely to try to avoid them. This leads me to suppose that someone avoiding violent games/books/films etc won't be traumatised by them in a PTSD sort of way because erm... they're avoiding them. If on the other hand we force everyone to experience all the hard core violent games/films/books out there then maybe we would be able to traumatise a few of them. We probably shouldn't do this