1. you sure your date and time on your system are correct?
2. if you not use internet explorer and edge on your windows than start it and try to clear its cache (do this when gog is turned off)
ok sorry to hear
check one more thing - do you have any space left on your system hard drive?
I can only guess that removing GOG and installing it freshly will fix problem and I don't think you need to remove game from drive so no redownload is required for you. But before you do this you can try to contact official gog support