GMP (short for Gross Military Product) was a currency used by the Militaires Sans Frontières and Diamond Dogs private military companies between the 1970s and 1980s. It's value is never compared to any specific currency, but money is presumably incorporated into its formula.
GMP was primarily used by Big Boss' mercenary companies as a means of pursuing research and development for military equipment. It was supplied by the Combat Unit, with the more skilled combatants providing more GMP.
During the 1980s, GMP was used by the Diamond Dogs to operate the Fulton recovery system, with both target recovery and cargo drops costing GMP. It also saw use in research and development for various weapons, equipment, and security materials for both Venom Snake himself and his various allies, as well as for expansions of Mother Base itself and its various Forward Operating Bases once they have gained territorial waters for development. Offline GMP can have a maximum of 5,000,000 and online GMP can have a maximum of 25,000,000, giving the player a maximum of 30,000,000 and offline GMP will always have a minimum of 10% of the total GMP, after enough time is enough to process the GMP transfer and offline GMP will always drain first in a transaction. Although it is impossible to have negative GMP value normally, if they spend enough GMP to drain their offline GMP and they go offline, Ocelot will notify the player if GMP value is not brought back to positive value, morale will drop and soldiers will defect. This is not usually an issue as long as the player as a stable internet connection. If their internet suddenly disconnects, they can sell material to bring their GMP back to positive value to prevent defection.
Behind the scenes
GMP is an in-game currency that first appeared in Metal Gear Solid: Peace Walker. It is used exclusively by MSF, assisting in Mother Base's establishment as an offshore sovereign state. In Peace Walker, players can gain more GMP by adding more soldiers to Mother Base's Combat Unit. The more combat experience these soldiers receive through Extra Ops or Outer Ops, the higher their stats become, and the more GMP is generated.
While absent in Metal Gear Solid V: Ground Zeroes, as there is no access to Mother Base or Fulton recovery, GMP returns in Metal Gear Solid V: The Phantom Pain for similar purposes as in Peace Walker. Although earlier sources implied that the Fulton was unlimited in supply and only required costs of GMP to use, this was changed in the 2015 demos, showing a limit besides GMP. The Gamescom 2015 Demo also revealed that when creating your own FOB, you are required to use a similar currency to buy the lot, called "MB Coins".
Collecting 10,000,000+ GMP will net the player with the achievement/trophy "Prosperity."