Reinstalling Gimp 2.8 into 20.04
Solution 1:
install an old Mint 18.3 on a new partition and just use its GIMP 2.8.16 on your files. I upgraded to the latest Mint and hated the GIMP 2.10 as I lost my iwarp feature I use on fender flares on car pictures. Tried in vain to extract the GIMP 2.8 off a Linux STUDIO disk I had and all the online varients but kept running into the same version check roadblocks. I'm happy with the 2.8.16 and will continue to use it as is. Throw all your pictures on a SD card and just work off that so as to make portability between versions easy.