It wasn't a GPMG - that stands for General Purpose Machine Gun and describes a gun designed for the army, which can be used in both the light and medium roles. The MG 81 was designed for aircraft, although some were transferred to the army towards the end of the war for emergency use.

Aircraft guns are lighter and faster-firing than army ones, but the combination of light barrels and a high rate of fire mean that they overheat very rapidly if used on the ground.

The MG 81 wasn't the fastest-firing aircraft MG to see service in WW2 anyway; that was the Russian Ultra-ShKAS, which fired at a phenomenal 2,700 rpm.

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Tony Williams: Military gun and ammunition website