You cannot play if your friend is playing any game from their library. But if you time it right (e.g., while they sleep or work), you have a free Steam account with GMod access.
: Look for reputable community creators or subreddits like r/ChoosingBeggars or r/GiftofGames where users occasionally gift games legally through the Steam store. free steam account with gmod
Sometimes, GMod is bundled in massive charity packs (like Humble Bundle). People buy these packs for one game and dump the GMod key online. You cannot play if your friend is playing