Remember you can assign multiple categories to an individual game and it's worth doing that if you're trying to make it easy for players to find.
Gaffer:
Title:
"GURPS - Kasserine Pass, US Army platoon holds off Nazi Panzers"
Personally I would put that in the Historical and possibly the Action/Adventure and Survival categories.
If you were looking at a Twilight 2000 game then that would probably be in Apocalyptic, Historical and the Survival categories.
If you were looking at a game about current day Private Military Contractors then that would probably be in the Action/Adventure and Contemporary categories
If you were looking at a game about Aliens style Colonial Marines then that could well be put in the Sci-Fi, Horror, Action/Adventure and Survival categories.
And so on. Hopefully you're getting the idea. The current classifications give you a lot more options - as was mentioned above, a few years ago there were about half as many classifications (I think) and a massive expansion was implemented to give GMs more options to select when setting up their games.