You can not just unpack an archive, Gimp in linux consists of many dependent packages.
I have a (k)ubuntu 18.04 (bionic) desktop which still comes with Gimp 2.8.22 as standard. The next one along 18.10 (cosmic) comes with Gimp 2.10.6
Trying to force a Gimp 2.8 installation in that latest version is almost sure to fail.
If you have the newer Ubuntu the easiest way is install a VirtualBox running one of the older Ubuntu's.
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edit: As an alternative you could try something like PuppyLinux that can run from USB memory stick http://puppylinux.com/
Does look like Gimp 2.8.x can be installed. https://puppylinux.org/wikka/gimp
Long time since I tried out puppylinux, be prepared for a (linux) culture shock.
There are other 'live' linux distros that will run from a CD/USB - Finding one that includes Gimp is the problem.
I have a (k)ubuntu 18.04 (bionic) desktop which still comes with Gimp 2.8.22 as standard. The next one along 18.10 (cosmic) comes with Gimp 2.10.6
Trying to force a Gimp 2.8 installation in that latest version is almost sure to fail.
If you have the newer Ubuntu the easiest way is install a VirtualBox running one of the older Ubuntu's.
---------------
edit: As an alternative you could try something like PuppyLinux that can run from USB memory stick http://puppylinux.com/
Does look like Gimp 2.8.x can be installed. https://puppylinux.org/wikka/gimp
Long time since I tried out puppylinux, be prepared for a (linux) culture shock.
There are other 'live' linux distros that will run from a CD/USB - Finding one that includes Gimp is the problem.