How come GoG games aren't supported by Xfire? My friend can pirate Fallout and Evil Genius, and it logs his hours, but I actually buy games from GoG and my hours aren't clocked. I know this doesn't mean much to people, but I check out what my friends play and vice versa all the time. So I'd like my friends to see that I'm into logging hours in... y'know, good games.

Any of 3 reasons:
1) They use different registry entries because they don't use the same installer (and can be added manually in Xfire) - Example: Fallout (if I remember correctly)
2) The exe file is name differently (and Xfire's config file need to be edited before the game can be added manually or automatically found) - Example: Broken Sword 3
3) Xfire only detects the Steam version which relies on Steam's weirdness with folders and whatnot (kind of a combination of 1 and 2 with added retardedness) - Example: Abe's Odyssee
Either way, the correct place to ask is actually at the Xfire board, more specifically, the games suggestion board. Make a new topic or find an old one for the game (the rules are to have a single topic for each game and a single game per topic), tell them that the GOG version is not auto-detected by Xfire (and wether or not it can be manually added). With a little luck more people feel the same way, the topic gets a few replies, and the Xfire team takes notice and has the GOG game included in the next update.

Regarding Miaghstir's second point, I'd say it's the most likely reason. Games from GoG are installed to a different directory than usual. Somewhere in the Xfire options you can manually add a game by navigating to its EXE file. Try that first.

It depends on the game, sometimes Xfire looks up the path through registry entries that the GOG installer doesn't create, which is the case with Fallout. I had a thread long ago where I tried to make GOG games work with Xfire (through modifications of xfire_games.ini, which is where Xfire looks up how to detect all games the program supports).
Back then I could assign an unused number to a completely unsupported game and Xfire users not having my modified file would simply see "unknown game" for my status, but after a while some update to Xfire disabled that trick and I didn't care enough to continue working with just the games that are supported but not detected correctly... given enough interest among the GOGers I can resurrect that thread in a slightly modified form (that is, games based on DOSbox and ScummVM won't be seen by friends, as well as some other games not natively supported by Xfire).
If the EXE is named differently, Xfire won't even accept a manual addition, you have to edit the ini file.