Just for anyone wondering. Gog galaxy couldn't find the dlc after the pruchase. I had to check / repair the installation, afterwards all dlcs were deinstalled and I had to reinstall them, but now it showed also Tyrants of the Moonsea, however not under this name but under a seemingly random combination of numbers.
Now it seems to be finde (haven't tried to play it yet though), just wanted to mention it, in case anyone is having the same problem and looking for a solution. And of course to let gog know in order to fix it.

Cheers