Try rebooting the laptop.
I encountered issues opening archived files in .zip or .dmg formats with older versions of macOS on a couple of our organization's Macs which were due to file system issues on the drive. Run Disk Utility First Aid on the drive (or better yet the hidden Container). Even if First Aid reports everything as "Ok" click on "Show Details" and look for any unfixed errors. If there are unfixed errors, then you will need to erase the whole physical drive and restore from a backup or clone.
Try reinstalling macOS over top of itself. If this doesn't work, then try a clean install by first erasing the whole physical drive.
Edit: I'm assuming you tried downloading another copy? Does this only happen with the Gimp .dmg or other archives? Since Big Sur is so new, perhaps you are encountering a bug with Big Sur.