Earlier today during Kojima Production's panel on the FOX Engine, the Metal Gear mastermind himself revealed that Ground Zeroes and The Phantom Pain are linked in the form of the fifth proper installment in his super popular stealth-action franchise.
There was still a bit of confusion coming out of the reveal, however, so Hideo Kojima took to Twitter to clear things up, saying:
"Ground Zeroes" is a prologue of "MGSV". 9 years after that event will be "The Phantom Pain". MGSV is constructed w/ prologue and main game "TPP". The game play demo I presented today is the opening of "TPP" which is tutorial that starts from crawling.
So what I gather from this explanation is that much like we saw with Metal Gear Solid 2: Sons of Liberty's opening tanker mission, Metal Gear Solid V: The Phantom Pain will feature an opening section titled "Ground Zeroes" that is strikingly different from the rest of the game. Make sense?