I can't remember where to put user fonts. There is a directory to put fonts to make them accessible portably, but I accidentally deleted that directory. I think the answer is somewhere on gimp.org, but I can't find it. And yes, I did search before posting.
Actually, forget it. The GIMP (strangely) detects any folder named /fonts/ and lists it in Preferences -> Folders -> Fonts.
Vintage!