Metal Gear Solid V: Ground Zeroes on PlayStation 4 will be free for PlayStation Plus subscribers for the month of June.
In Japan and Europe, the offer is valid from June 3rd until July 7th. In the US, the promotion begins on June 2nd. Keep in mind that the offer is only for the PlayStation 4 version of the game.
Source: Metal Gear Official Twitter Japan, Sony Japan, via DualShockers, Konami Europe