I'd like to use Gnome on Qubes-OS, but Gnome GDM does not obey standards and so cannot be used.

Per the Qubes-OS website (https://www.qubes-os.org/doc/devel-faq/): 
Qubes requires KDM (KDE Login Manager), rather than GDM, for the very simple reason that GDM doesn’t obey standards and start /usr/bin/Xorg instead of /usr/bin/X. This is important for Qubes, because we need to load a special “X wrapper” (to make it possible to use Linux usermode shared memory to access Xen shared memory pages in our App Viewers – see the sources here). So, Qubes makes the /usr/bin/X to be a symlink to the Qubes X Wrapper, which, in turn, executes the /usr/bin/Xorg. This works well with KDM (and would probably also work with other X login managers), but not with GDM. If somebody succeeded in makeing GDM to execute /usr/bin/X instead of /usr/bin/Xorg, we would love to hear about it!