The Steam pages for both MGS 5 and Ground Zeroes are live.
Konami spoiled their own Gamescom surprise today.
The UK Konami website put up an article saying that Metal Gear Solid V: The Phantom Pain and Ground Zeroes were heading to Steam. Once clicked the article lead to a page not found, but it's been made official by Hideo Kojima himself during the MGS livestream.
You can catch the rest of the livestream here: