With Mint being based on Ubuntu and Ubuntu based on Debian you would hope that there might be a backward path.
The Debian packages are still there, see:
http://packages.debian.org/squeeze/gimp If you go to the bottom of the page, choose your architecture which takes you to a list of mirrors you can download the deb file.
I did give it a try for you in a LM14 VM, starting with the basic gimp, gimp-data & libgimp packages.
Assuming a terminal in the directory with the packages.
The command (as root) is dpkg -i --force-overwrite *.deb
but this quickly leads to dependency hell & downloading the required packages brings up further dependencies and I lost interest.
Another way is to add the squeeze repo to synaptic.
The line
deb http://ftp.debian.org/debian/ squeeze main contrib non-free
works.
Then in synaptic you can force a package. Ended up uninstalling half of Mint so that is no good.
I suppose it might be possible to compile Gimp 2.6.x against more recent dependencies (babl, gegl, python etc) but I dont know and think you will be in serious risk of breaking your installation.
A better bet might be find an old-ish live CD/DVD which includes Gimp 2.6.x and install as a parallel OS or maybe as a VM.