You should be able to do this with a combination of startx and /etc/X11/Xsession, as long as you've got the necessary trees bind-mounted (I'd guess that you'll need /dev, /sys, and /proc)
Chroot into your… chroot and run
sudo startx bash
which should give you an X term with a shell. To start a GNOME session you can simply run
/etc/X11/Xsession
from that shell.
You should be able to get this on the VT of your choice; check out the man page for startx for this, and other potentially interesting options.
Note that this doesn't make for a secure chroot - since /proc and /sys and /dev are mounted in the chroot a sufficiently advanced malicious user break out of the chroot with ease.
/mounted? with chainroot command you can pretend your/is temporarly somewhere else, which allows you to run "bootstrapped" debian/ubuntu releases. – Dima Aug 16 '10 at 01:31