I’ll put it simply: Someone ported the original DOOM into Garry’s Mod, and it’s amazing.
A user by the name of GhorsHammer has taken the original DOOM, and partially ported it into Garry’s Mod. What does this mean exactly? Well, you can now use all your favourite DOOM weapons on Half Life 2 enemies, or pit off against DOOM enemies within the Source engine.
Or, if you’re feeling really creative, you can make a Strider fight a Spider Demon.
Just watch the video, you wont be disappointed.
A download of the mod can be found here.