These posts are several years old, but I thought I would mention here that I am adapting Butch Curry's (posting on Cartographer's Guild as palehorse and ZombieButch) video tutorial series to GIMP. I am currently on Episode 6, which uses this woodblock method, so coming across this thread was very helpful! If you look through the Tutorials section of the forums, you should find my thread for the adaptation. I hope you find it useful.

PS: As usual, I will be giving credit and kudos to RobA in my adaptation for this extremely helpful technique!

wisemoon