Well this is interesting news. The Metal Gear franchise of tactical espionage games has traditionally been on game consoles as far back as the NES. I remember seeing their Revengenge game hit Steam and thought, man, it would be awesome if they released those other Metal Gear games on Steam as well. Well, it may not be all of them but this article confirmed that part 5 is coming to Steam. If you've never played a Metal Gear Solid game, you really should give it a shot. If not for the action, do it for the story telling. Lots of nice dialogue and voice direction in these games. Metal Gear Solid 5: Ground Zeroes is a prequel to the Phantom Pain. If they end up listing both seperately, it's best to just get Phantom Pain since it will have more content than Ground Zeroes.
Ground Zeroes from my experience is a one map game that helps fill in a bit of story to hype the Phantom Pain. It does a good job of it but I'm sure you want more bang for your buck.