I believe that x11 was a custom install. I have gimp 2.6.6 working on my iMac g3 600.
Might be correct version:
Restore Tiger 10.4 & Leopard 10.5 DVDs are available from Apple by calling 1(800) 275-2273. Have your serial number ready. Have your credit card ready too. There may be a small fee.
https://discussions.apple.com/thread/4720126?tstart=0 -- January 20,2013
https://discussions.apple.com/message/24588313#24588313 -- January 22,2014