As the name implies, Metal Gear: Doom is one modder's preliminary attempt to convert Metal Gear—Hideo Kojima's 1987 series debut—for the Doom engine. For those not in the know, Metal Gear isn't the 1998 PlayStation game, but rather the original top-down 8-bit game which released for NES, and originally the MSX home computer.
Created by Doom modder AutumnAPM and in development for at least three years already, there's currently a playable demo on Moddb but it's really just "snippets of multiple aspects of the project". Still, the screenshots alone provide a nice overview of how the mod will look: the textures are very much in keeping with the original 8-bit aesthetic, as the image above demonstrates.
Here are some other images (you can check out some videos on the ModDB page):
"The map is me testing textures, [and] seeing pixel scalings from the MSX/NES primarily," AutumnAPM writes in the comments. "UI is a very early version, items will be used and the guns are unfinished.
"Currently the focus of the project is to make a map pack for doom themed on metal gear then work it from that into a metal gear game on the doom engine."
Cheers, DSO Gaming.