to tell you the truth, i tried several times with a clean install of pc-bsd and it always failed. Im pretty sure there is a way but the dependencies never matched up.
Anyways if you want gnome it is mush easier to install freebsd do a port update and then do a pkg_add gnome2. This is the easiest way i found to have gnome running.
you have to be logged in as root to do this
# portsnap fetch extract
# pkg_add gnome2
# vi /etc/rc.conf
# i gdm_enable="YES" ( i tells vi to go into editing mode & add this somewhere in there )
# :x ( saves the file and exit vi )
reboot system and it should start gnome auto, that being that you have a correctly configured x server, which is also pretty easy to figure out