The latest system update pulled GDM 3.2.x into my ~amd64 Gentoo system. While GDM 2.x felt just right, GDM 3.x is not my thing at all. To bring back GDM 2.x I added these masks

>=gnome-base/gdm-3
>=gnome-base/gnome-control-center-3
>=gnome-base/libgnomekbd-3
>=net-wireless/gnome-bluetooth-3
>=gnome-base/gnome-settings-daemon-3

and re-emerged the latest 2.x versions of these packages. Works.