Unlimited money, relatively quickly. If only real life worked that way.
Apparently, you don’t have to grind for GMP in Metal Gear Solid 5: The Phantom Pain. Especially towards the second half the game, it gets extremely grindy, with progression being tied to the player upgrading the Mother Base’s capabilities, which in turn, requires the game’s in game currency of GMP.
However, you don’t have to grind for it all after all- YouTuber BiigMoney figured you can use mines or decoys with a fully upgraded Fulton to make some easy cash, earning some quick money in the process. You need to have completed Episode 8 before you can utilize this cheat, and it takes a fair few minutes on each try, but I imagine that, on the whole, it will be preferable to most people than the endless grinding that the game will otherwise entail.
You can see for yourself how it is done in the video below. And if you plan on acting on it, you better do something about it now- you know Konami will issue a patch to remove this vulnerability sooner rather than later.