Not to pry into details, but is he into one specific game, or several different ones? I'm an avid gamer myself, but what I've noticed is that the addiction (or at least behavior I've observed in other gamers that seemed to fit that description) seems almost more to the social aspect of the game, the dependency that players form on each other. In MMO-type games in particular, players often specialize in a few specific areas of strength, requiring them to work together. This means that in your BF's mind other people DEPEND on him to play, or at the very least they respond to his play in a way that he finds stimulating (he could well be the villain as well, but thats another side of the same coin IMO).

This is going to sound trite, but have you attempted to engage him in some type of gameplay? I wouldn't suggest that you actually join in his addiction of choice, if it centers on one socially active game in particular (World of Warcraft would be a popular example), but you may be able to find something that he likes which is cooperative on a smaller scale, and play that together, just the two of you. Obviously this will not reduce his actual gaming time in any significant way, but it might demonstrate to him even more than your other efforts to be accommodating that you aren't trying to take away something he enjoys, you just want to be able to spend time with him. The problem is that even just saying "I want you to put a time limit on it" just brings up a natural defiant response, like any addict that is told they must cutback their drug of choice. If you take the step of trying to game with him however, that might impress upon him just how seriously you take your relationship, and get him to reflect on why he is with you in a positive light.

To boil down all that rambling, Baltar-style: You have used the Stick. It may be time to try the carrot.