gimp 2.6.3 is not offical supported, what will not say that you can't use it.
first go to www.getdeb.net
and download the new version of gimp.
you must download all the packages file per file, place them all in the same directory without other files
ex: gimp , libgimp2.0 and gimp-data
for Elyssa the Herdy packages and for Felicia them for Intrepid
now go in the folder where you have you new gimp packages that you had downloaded from getdeb
-open in terminal and type:
sudo dpkg -i *.deb
so you can enjoy the last version of gimp
do not install them by double-clicking the packages, because than the risk that you became dependences problems or break something is bigger