Comment # 1 on bug 1091516 from Richard Wallwork

I have exactly the same issue as this on a 13.3" 1920x1200 laptop. gnome-shell
and GDM are scalled 200x on first boot which makes text huge and the UI almost
unsuable.

Easy to change the settings for desktop but to resolve GDM had to log in as GDM
and use gesttings to change to 1x scaling.