GDM is for GNOME Display Manager, the graphical login program in GNOME. I recently (though it's been a couple of years I started using GNOME. Well I actually started using a touch pad recently :-) ) found that mouse tap is not working there. Called google and the was able to solve the prob. Here's what should be done.

Copy the file /usr/share/hal/fdi/policy/20thirdparty/10-synaptics.fdi to /etc/hal/fdi/policy/

Open the file and you will find something like

<!-- To add custom options for the touchpad, modify the examples below
to suit your needs. The available options are listed in the
"synaptics" man page. After modifyfing this file, you must
restart HAL. Check the output of lshal whether your modifications
have been merged successfully.

Note: Options must always be type "string".
The following examples enable left, right, middle clicks on
single, double, triple finger tapping, respectively.

<merge key="input.x11_options.TapButton1" type="string">1</merge>
<merge key="input.x11_options.TapButton2" type="string">3</merge>
<merge key="input.x11_options.TapButton3" type="string">2</merge>
-->

Just below that add the following line


<merge key="input.x11_options.TapButton1" type="string">1</merge>

Save the file.

Now you need to restart hal. You can simply reboot the machine or

Mouse tap should work now.