How to use custom ~/.xsession with gdm3 in Ubuntu 18.04 Bionic?

Question

I upgraded from 16.04 to 18.04 and can no longer choose my custom ~/.xsession in the session menu in gdm3 or lightdm. If I use xdm as the login manager my ~/.xsession runs, but then I have other problems related to Gnome (my custom ~/.xsession runs Gnome + XMonad).

How can I use my custom ~/.xsession with gdm3 or lightdm?

Solution

I can run ~/.xsession with gdm by creating /usr/share/xsessions/xsession.desktop containing

[Desktop Entry]
Name=XSession
Comment=This session uses the custom xsession file
Exec=/etc/X11/Xsession
Type=Application
DesktopNames=GNOME-Flashback;GNOME;
X-Ubuntu-Gettext-Domain=gnome-flashback

and then selecting XSession from the sessions menu when I log in.

(I wasn't able to find a package containing such a file, but it's easy to create it yourself once you know what to do).