Seems like a great idea. I can think of a few enhancements though that might prove extremely useful to add more functionality to this program.
I would suggest an advanced/basic interface option. The basic interface option could solely derive it's information from the info file to show the mods name, version and description. Clicking on it gives some simple options, add/remove.
This could easily become an entire add on manager. This could insure complete and proper removal of add ons that may require files placed in data folders, maps, models, etc. This could also have an ability grab status updates from each add on authors web site. E.g. wire mod hud display is busted due to latest gmod update. The program could update when ran and check for any known issues or available updates.
It could be like a svn update system in that respect, except only handing out release versions.
A database of available mods could also be placed (like
www.garrysmod.org) into the manager, allowing people to download and install them with just a click of a button.