i'm not too sure on the exact steam program id (as i haven't got garry's mod installed), so you'll have to do a bit of work.
in the steam library, right-click on garry's mod and create a desktop shortcut. right-click >> properties and you'll see something like steam://rungameid/4000
- you may have a different number. copy and paste the whole thing. you can then delete the shortcut, we just needed the right id.
next, open notepad and type (or paste):
start /high steam://rungameid/4000
then save it as garry.cmd
, make sure you don't save it as garry.txt.cmd
save the command file (and a shortcut) wherever you like.
i think that'll work.