Single Status Update
Apparently GOG Galaxy broke when it comes to fetching games. I'm using the old version, though I wouldn't be surprised if this happened in the new version.
I discovered this when I decided to reinstall TIE Fighter only for it not to show up, despite me knowing for a fact I had purchased it and previously played it. But it doesn't show up at all in searches of the library in Galaxy. On the website it does show up, and when checking this out I noticed that I had literally one hundred more games listed on the website compared to Galaxy.
I'm 90% sure I've found the issue. Anything sold in a bundle will not show up on Galaxy. So TIE Fighter didn't show up because GOG actually bundles the original game, the CD rerelease, and the 1998 "special edition" remake using the X-Wing vs. TIE fighter engine. Similarly none of my Space Quest games show up since they are sold in a 1-3 bundle and a 4-6 bundle. Anything with more than one entry to the store purchase won't show up.
It had to have happened recently, since I successfully installed SWAT 2 about a month ago and that comes in a bundle with SWAT 1 (and neither game shows up in my galaxy library at the moment.) DOOM and DOOM II even vanished from the galaxy version of the library due to the enhanced editions of those games being given to people who had previously purchased them.