I've just spent three days looking for Frostcrag Spire only to find that it isn't in the GOTY edition sold here on GOG.

Can I buy it anywhere, or is my game hoped? It's really the only stronghold I want.

It is included, but not enabled by default. If you start via the launcher (instead of starting the game directly), you have the option of selecting what plug-ins are enabled, and you can then enable Frostcrag Spire. (I know because I enabled it and used it heavily when I was playing Oblivion.)