As written in the topic GoG Galaxy consumes disk space from C: Drive on its own I had 40 GBs after a few moments the disk was full only removing GoG galaxy helped. I dont know whats causing it but I thinks its pretty serious bug.

I see files accumulating in C:\ProgramData\GOG.com\Galaxy\support for some reason. That's probably where your space is going.

 
I'm not using Galaxy, but there was a known issue with its debug log.  Could it be the culprit?

Make sure that Windows is configured to <span class="bold">show hidden files and folders</span>, and take a look at the folder " C:\ProgramData\GOG.com\Galaxy\redists\ ".  There's probably a huge file named " debug.log " in that folder, and you can safely delete it.

More details in <span class="bold">this thread</span>.

I didn't experience this problem, but some people claim that the culprit is a file called debug.log, in the galaxy folder.
Go to the directory, and search for the file in the search bar.
It definitely shouldn't be 40gb, mine is 0kb.