I think this is not the best approach. It produces a standalone and it
removes one of the things that Qubes has worked hard to provide - the
functionality between qubes.
Do this instead -
In the template you want to use, install tigervnc-standalone-server and
tigervnc-viewer.
Create a script at /etc/skel/gnome.sh with this content:
#!/bin/sh
tigervncserver -localhost -xstartup /usr/bin/gnome-session -geometry 1024x768 -SecurityTypes None -useold :1 && xtigervncviewer 127.0.0.1:5901 &
Make the script executable - chmod +x /etc/skel/gnome.sh
When you create a qube using this template, the gnome.sh script will be
at /home/user/gnome.sh
You can use that qube normally, with full access to qvm-* tools.
But you can also create a shortcut or menu item to run qvm-run QUBE /home/user/gnome.sh
This will automatically spawn a gnome desktop, again with full access to
the qvm-* tools.
(An advantage of tigervncviewer is that you have a control menu at F8 which
allows you to run up to full screen.)