There are millions of gun otaku, or lesser aficionados, all around the world, so preferably game studios would actually want to use models and names of real guns. In the past they more readily did. They even sometimes paid the arms manufacturers some nominal compensation for the trademarks. I recall EA made a while ago a decision to drop the payments but to continue using the models. The gun makers haven't always been happy with their products appearing in games because of the negative image shooters have accumulated due to populistic politicians and non-governmental organizations demonising games to further their own agendas. However, I imagine things are a bit different in Japan due to their draconian copyright laws. GGO is supposed to be American, though, so in that sense real gun models in the game wouldn't be out of the question.
In short, lots of people like to see real brands and models in games. I'm not much of a gamer or gun fanatic, but even I like to see guns I recognize, instead of totally fictional ones. Non-MMO PC MP games invariably get real guns introduced as mods, which tells something about their popularity.