Change user's window manager:
https://unix.stackexchange.com/questions/317591/gdm3-change-default-wm-from-gnome-to-i3
(but edit Session instead of XSession).
E.g.
[User]
Session=openbox
There is also ~/.dmrc advertised (https://help.gnome.org/admin/gdm/stable/configuration.html.en)
but this has not worked for me.