You should be able to do this with a combination of
/etc/X11/Xsession, as long as you've got the necessary trees bind-mounted (I'd guess that you'll need
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
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
/dev are mounted in the chroot a sufficiently advanced malicious user break out of the chroot with ease.