How to start GNOME session from chainroot?

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.

I have used such a setup a long a time ago. I believe it was just as simple as starting X from the chroot specyfing a different display name:

startx -- :1

Switch to it with: CTRL-ALT-F9