After some email exchanges with the freebsd-ppc mailing list, I hit on the solution to the problem with the gmp port:

edit the port Makefile to add the following CONFIGURE_ARGS:

--enable-shared
--build=powerpc

This gets past the configuration failure and allows a successful build and install. So I'm off and running with the rest of my setup!

Don't know why this didn't work out of the box, but I've let the port maintainer know so he can followup.