I've just read about the Desecrated Lands expansion for HOMM2 and wondered if there is any chance it will come to GOG?

I don't think so. It's not really an expansion, just a pack of unofficial and fan-made maps. Some of these maps were collected from the internet, so I suppose are still available out there as single maps.

It was released as a free add-on to some CD cases. I see a used CD is currently going at 430$ on amazon. As with all fan made maps, the quality and balance might be questionable in some.

I doubt you'll ever find an official release anywhere, but you can easily find the whole thing on abandonware sites. You know the discussion about the thics of this yourself.