Unfortunately, a handful of games are still marked as incompatible with GOG Galaxy. What this means is that they can't be installed via GOG Galaxy, and they don't use features provided by the client.
However, you can still use Galaxy to download the standalone installer. Simply go to your library (click on LIBRARY on the top bar), select the game you want to install, and click on MORE -> Backup & goodies. You will be taken to the Backup & goodies page. Standalone installer will be available under GAME BACKUP COPY section.
After the standalone installer is downloaded, you can find it in your downloads folder (by default C:\Program Files (x86)\GalaxyClient\Games\!Downloads). To install the game, simply launch the installer and follow on-screen instructions.
Alternatively, you can download the game from our website. Just open your Account page, select the game you want to install and download all parts of the offline installer to the same folder. After all parts are finished downloading, launch the executable file.