GURPS can handle almost any genre. I'm known to say, "GURPS can run any campaign up to but not including truly omnipotent gods (for that, use Nobilis)." You can run a game where the PCs are mice fighting to survive in a war-torn pantry. You can run a game where everyone is a software personality emulation, trying to debug a new world on a virtual server. There's practically nothing that GURPS can't run.
But I'd say the genres at which it truly excels
1. Are focused on the human scale (both physically and emotionally/mentally),
2. Blend heroic adventure and realism (as seen in most fiction), and
3. Reward planning and tactics (as opposed to, "The PCs should win because they're the PCs!").
For any genre described by all three of those, I'd argue that GURPS is the RPG system to handle it, even over a system designed specifically for the genre.
For any genre described by one or two of those, I'd say GURPS will still do an excellent job at it, requiring minor tweaks at the most. A specific, dedicated system might be better, but I can think of actual examples where that's not been the case. (No, I'd rather not elaborate; this thread isn't about tearing down other systems.)
For any genre that doesn't match the above at all, GURPS can still handle it. In fact, it's entirely possible that GURPS will handle it better than any other system around! (As vm points out, while GURPS doesn't necessarily excel at mixing mice, human, and elephant PCs, it arguably does a better job than any other system out there.) But you may have to make some significant tweaks, buy an appropriate supplement or two, or just be comfortable making a few GM calls.