hey!
I bought the expansion pack heart of stone several months ago and installed it back then, too. now I wanted to replay the game and decided to play in german (mothertongue and stuff ;)). I couldn't change the languagesettings so I read that for some people un- and reinstalling "heart of stone" worked out to be able to play in german. so I uninstalled it. Now I somehow can't reinstall. :( it's listed with my orders but there's no download button. Also, when I search for the game through the GOG browser, the only option I have is to buy a new one. I seriously just want to replay this great game :D
thanks in advance

Did you by any chance buy a Witcher 3 key/gift code from a key reseller?

Anyway, your best course of action is to contact GOG support to get a reliable statement about the actual situation (technical problem, illegitimate key, or something else) and how it will or should be resolved.