Anybody else with this problem?

I see it on the web browser and Galaxy. Where are to trying to download it?

Have you tried maybe cleaning out your browser's cache?

1. Clear the cookies in your browser for gog.com only.

Firefox - Clear Cookies for a Single Website

Chrome - Clear Cookies for a Single Website

Microsoft Edge - Clear Cookies for a Single Website

2. Close your browser.

3. Flush Your DNS Cache using either the Run dialog box or CMD / Command Prompt.

4. Reopen your browser.

5. Reopen https://www.gog.com.

6. Log into your GOG account as normal and access your game library.

7. Try to download the relevant GOG offline installer(s) for the game(s) you want.

SOLVED

Turns out it was my ublock filter, which hides the giant "Download Galaxy" button that is displayed in every game page. Except for this game O_o. So the silly filter !www.gog.com##section.game-details__section:nth-of-type(1) blocked the download section intstead of a nonexisting galaxy ad.

Nevertheless, thanks for the help.

Try replacing it with this:
gog.com##.game-details__section[ng-if^="details.simpleGalaxyInstallerPath"]