Nope, there's no option for that in phosh settings. Actually, it's the compositor (phoc) that starts gnome-session in /usr/bin/phosh. I've just tried to edit the startup script to launch gnome-session directly, but it would refuse to start.
It's also a pure wayland system, so it's quite unlike what we're used on desktop.
It's also a pure wayland system, so it's quite unlike what we're used on desktop.