Enabling tapping support in Gnome 3, I still don’t have support for that at the gdm 3 login prompt .
In /etc/X11/xorg.conf.d/50-synaptics.conf I got tapping support for my mousepad at the gdm3 login prompt by adding
Section "InputClass" Identifier "touchpad catchall" Driver "synaptics" MatchIsTouchpad "on" Option "TapButton1" "1" Option "TapButton2" "2" Option "TapButton3" "3" EndSection
The file wasn’t there, so I created it.