### be a gdm user
sudo -u gdm -s /bin/bash
### set gdm default font
dbus-run-session gsettings set org.gnome.desktop.interface font-name 'Terminusa 12'
# or
dbus-run-session dconf write /org/gnome/desktop/interface/font-name "'Terminusa 12'"

### set touchpad tap to click for gdm 
dbus-launch gsettings set org.gnome.desktop.peripherals.touchpad tap-to-click true
# or
dbus-run-session dconf write /org/gnome/desktop/peripherals/touchpad/tap-to-click true