It's not Hot Topic and GameStop that are merging; it's GameStop and ThinkGeek. There is the opportunity for "synergy" (yeah, I hate the word, too) in that they're both retailers to a similar demographic, but one is online and the other is brick-and-mortar. They can do all kinds of cross-selling and get a combined company that sells more than either of the two separately. As well as eliminating a few minor redundancies in the staffing (such as maintaining two completely independent web sites), though that kind of thing never produces really huge gains.
That's an *opportunity*, of course, not a guarantee. They can still fuck it up royally. In fact, that's where I'd lay my money.