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It's basically an Air Traffic term for 'Ground'.
As you go from quieter to busier airfield with more controllers, the responsibilities for the aircraft are divided between more ATCOs.
-At a small airfield, you might have 'Tower' only, who does everything.
-Getting busier you might have two ATCOs, Ground (we call it GMC) and Tower (we call it 'Air').
-Busier again you might have Clearance Delivery (We call it Ground Movement Planner - A/c call here first to obtain their clearance), GMC and Air
-At Heathrow we have GMP, GMC1, GMC2 and two Air controllers, one for the take off runway (Air Deps) and one for the landing runway (Air Arrivals).