Advanced Options on the Create a New Image dialog box appears below buttons and flows off the dialog box where some of its options cannot be seen. See attachment.
I installed Gimp on OS X 2.6.5 and X11 126.96.36.199 on OS X 10.5.6.
You can contact me at gil.yoder @ oabs.org. I would be glad to be of any assistance, if you need my help to reproduce.