Ok, to me someone who is a trendy gamer, is one who jumps on the bandwagon of gaming while it's "in". Like the people who play Halo or CoD exclusively. It sounds like he was calling you a gamer but using trendy not as an adjective of gaming. Because if he was, then he's an idiot. Unless he caught you buying a copy of Halo, then I can see where the inference comes from, again, not that it's correct.
That sucks too, because it's a dickish thing to say. Even trendy people cringe when being called trendy. It has negative connotations the way that "preppy" does. How about "fashionable gamer"? "Abercrombie & Gamer"?