app-gnustep/gnumail, as it currently exists in portage (as taken from bug 14132), does not include support for the optional Clock, Face, and PGP modules. There isn't really a USE flag that would apply for Clock, but Face is covered by xface and PGP is covered by crypt. The ebuild atttached to bug 18075 does include support for these optional components, but doesn't use the gnustep ebuild class. Perhaps the relevant parts from bug 18075 should be merged into the ebuild. Additionally, I've successfully built everything from app-gnustep/* under ppc, so ~ppc should probably be added to the KEYWORDS of those ebuilds.
gnumail-1.1.0_pre2-r1 is now in portage with xface and crypt use variables. Please test this build and let me know if things work correctly for you.