Bought it. Not showing in my library to install. On store page it says "In Library", but it isn't. I've did a search and I've cleared filters and I've went to Newest first and Purchase date, it's just not there.

I get this with every purchase through the GOG Galaxy client.
Close down the app and relaunch it, the game should be there after that.

If you access to your library from the browser, delete the cookies and reload the web. This usually happens with new purchased games and DLCs.