I do that, but clicking on any of the settings doesn't allow me to change anything.
EDIT: Nevermind, apparently it takes a second from clicking on it once to go into edit mode.
Instead of clicking-waiting... or slow-double-clicking... or half-clicking-mostly waiting... or clicking-wait-click... or click-half-wait-click... etc
Just highlight the GMST and press F2 on your keyboard.
It will go into "edit" mode.
This works in almost all other situations in the CS when you need to edit a highlighted field. Like cells or IDs.
Also,
http://www.scribd.com/doc/7007668/Morrowind-Scripting-for-Dummies-9 has a section on the GMSTs. It explains what most of them do, and how the variables work. (some are unknown, but this is the definitive guide)