I've noticed that some games available on steam will continue to run even if the underlying steam account is logged out for some reason. This can either be from a bad internet connection or from someone else on another computer logging into the same steam account.
These are typically older games that have no steam integration. I know this is the case with Trackmania United.
So it might be possible with older release games or third party games to get by the restriction by lauching the game on one computer, then logging into steam with the same account on another computer. If the game doesn't use steam for finding matches (and uses some other method) than it might work. That being said I am not advocating the 'theft' of games from steam. Particularly since your example game Garry's Mod only costs $4.99 right now.
You could always gift it to her account :)