I am not positive hear, but I think sudo systemctl disable phosh and rebooting will get gdm to display properly.
Since Phosh doesn’t use a Login Manager, I think what is happening is that gdm is starting up and then Phosh is starting over top of it.
To be clear though, I am not sure how well gdm/GNOME will work though and you may need to re-enable and re-start Phosh if you want the phone to go back to normal. (and maybe also disable gdm)