A Gatling Gun is a fixed weapon that discharges rounds in rapid succession. It occupies a 4x1x1 space. In Creative Mode it has infinite ammo, but in Survival it must be loaded with 25x184mm Ammo Containers via the one of the two access points on the sides. It can hold 4 full ammo containers, which is enough for approximately 10-12 seconds of continuous firing.
The Gatling gun was a mid-19th-early-20th century rapid-fire weapon, forerunner to the modern machine gun. It consisted of a crank, which rotating six barrels around a central shaft; each barrel firing once every revolution in roughly the same position. After it was replaced in service by recoil- or gas-operated weapons, the concept fell out of use for many decades. The basic design resurfaced, however, after World War II, with the development of the M61 Vulcan, a pneumatically- or hydraulically-driven, six-barreled, air-cooled, electrically-fired 20mm Gatling-type weapon.