I've seen some very awesome and artistic Garry's Mod screenshots on Facepunch, so it's definitely a possibility, i dunno what your friend and DeviantART users are talking about...
Edit: I tried making a Left 4 Dead kind of thing only with Half Life 2 stuff, like the First Aid Kit being the Med Kit from Half Life 2, the Pistols being the 9mm Pistol from Half Life 2 and the Shotgun being the Shotgun from Half Life 2