OK it turned out the gmd3 login prompt dialog wasn't actually absent... the issue was due to my install actually thinking the machine had two heads, when it didn't (7700K i7; CPU's integrated graphics on an Asrock H270 Pro4 mobo, one monitor attached to VGA port... but the freshly installed system seemed to think there was a monitor on one on the other ports too). The dialog was presumably on the invisible-to-me display. On closer inspection I could slide the mouse off the left hand side of the screen I could see. I think lightdm appeared to work better as it has he login dialog follow the mouse from screen to screen (maybe gdm3 would be improved by such behaviour; couldn't see anything in its config to select anything like that though).
Anyway, the two screens issue became apparent when I looked closer at the desktop lightdm was logging me into and I was obviously looking at the secondary screen of a primary+secondary pair. Reconfiguring displays (usual right click on desktop) to be just a single primary on the desktop I do have worked good. The trick was then to copy the logged in user's .config/monitors.xml over to where gdm3 could use it: sudo cp .config/monitors.xml /var/lib/gdm3/.config/ sudo chown Debian-gdm:Debian-gdm /var/lib/gdm3/.config/monitors.xml And after a dpkg-reconfigure gdm3 and selecting it as the login manager, and a system restart, I'm seeing the expected gdm3 login prompt. Phew! (Makes me wonder if the Thinkpad I also ran into this on had the same problem.) Will send a separate control email to re-close this issue.
