gmDoom is what happens when you combine Half Life 2 with Doom. Through the magic (or technology, in any case) of Garry's Mod, a modder going by GhorsHammer is putting together a source port of Doom integrated into Garry's Mod 13. Watch it below:
The mod puts players in control of the Doom Guy and allows them to play through the entirety of Half Life 2's single-player campaign with weapons and gear from Doom. Not only does it look like Doom, it also plays like Doom, so you won't have to look up or down to take shots at the enemies. Even the game's heads up display is tweaked to look like the id Software classic.
According to its creator, the assets are loaded directly from an old IWAD file, and no binary modules are required so you won't need to do much to get the whole thing up and running.
gmDoom will be available as an add-on in the Steam Workshop once the modder irons out the bugs.
