The Galactic Federation Marine Corps is a military group of the Galactic Federation that dispatches Federation Marines to planets in need. Marines are specialized in air and land combat and often travel in squads. They are first mentioned in the manual of Metroid: Zero Mission, and play important roles in Metroid Prime 2: Echoes, Metroid Prime 3: Corruption, and Metroid Prime: Federation Force, although they weren't specifically called the "GFMC" in the latter. A group of Marines is also seen fighting Sylux on what appears to be Cylosis in the introduction video of Metroid Prime Hunters.
Marine Trooper Types
Normal Trooper- The most basic unit of the GFMC, They use basic power-beam weapons and are the main fighting force for the GFMC.
Demolition Trooper- Marines that specialize in explosives. They carry a Det Pack on their backs and have a yellow shot from their guns. They have the weakest armor and in Metroid Prime 3: Corruption, they helped Samus blow through some obstacles.
PED Trooper- When the Marines began experimenting with Phazon they developed a new kind of soldier with enhanced weaponry and armor. They are extremely powerful and are invincible for a short period of time when they activate their PEDs. Upon the destruction of Phaaze, these soldiers were presumably decommissioned (as Phazon no longer existed without Phaaze).