I don't use gnome, so can't speak to that problem.
For a regular user to
su to root, you'll need to add him to the wheel group. This is the default on FBSD 6.x / 7.x (not sure about before then) systems, and is controlled via pam:
Code:
%cat /etc/pam.d/su
...
auth requisite pam_group.so no_warn group=wheel root_only fail_safe
...
Please see my signature for a couple prominent FBSD learning resources.