the Gnome Desktop Manager ( GDM) is in a state of being rewrote from the ground up .
The upshot of this is that gdm in fedora 11 is ONLY a partly working svn build and only barly just works .
so the short answer is NO you can not do that using GDM
however you CAN use KDM to start Gnome
add a /etc/sysconfig/desktop file
with this text in it