Because garrys mod is on the source engine.And the source engine can run on just about anything.
In my opinion the Source Engine is getting a bit outdated. Valve really needs to step it up considering all the new advances that are being made in the industry.
they should spring for a 3rd person game
What video card you have, you need nVidia or ATI.
Just put in your admin pass, or change the settings so that you don't need administrative premission.
I am an admin on my laptop!