Sorry guys, the release I've made had a small (or should I say "huge") bug. I tried to autodetect the dpi setting X11 is using. Unfortunately I forgot to test for non existing values. So it came, that if one uses a vanilla Leopard X11 where no dpi value is set in org.x.X11, the dpi variable in the autodetect script was empty and so a font size of 936.0 was calculated. Rather huge.
I've just added some testing code for this situation. Now it should fall back on 96 dpi, which is the default value of a vanilla Leopard 10.5.8 X11. The font size will be 9.8.
A new release has been uploaded and is waiting for you.
So, please, re-download the "Gimp-2.6.7-p1-Leopard-universal.dmg" from http://sourceforge.net/projects/gimponosx/files/
and edit your "~/Library/Application Support/Gimp/gtkrc" like shown above or delete the file "~/Library/Application Support/Gimp/gtkrc". GIMP should now autodetect the correct values.