Bought this earlier this morning. My GOG Galaxy has been trying over and over to install the game, but it keeps failing. Disk still has 436+ GB free, internet connection is stable (I mention this because I'm living in Indonesia, where that's not always the case). I don't use Avast, so it can't be interference from that antivirus.

Never mind. Downloading the installation files separately and then installing them to GOG Galaxy worked.