I know it's been already asked, but: is it included in GOG's version of the game?

Reason I'm asking is that both in Steam (where the Lindworm is a free but separate DLC) and in GOG the storage requirement is stated to be 1500 MB for the main game. Since Steam's page of the DLC states its requirement is 15 MB, shouldn't GOG's version take 1515 MB if the Lindwurm is included?

It says so in bold letters on the game's page.

Either that message is new, or I totally missed it to begin with. Well, anyway, that's good news.

Yes.