The Gatling Gun appears to be a GAU-19 that has been modified to be hand held, it has three barrels, is fed from a 400 round drum and has a rate of fire of several thousand rounds per mminute. This is the weapon of choice for Gatling Superheavies and it is the only source of Gatling Guns in the entire game.
The Gatling Gun seems to use an inefficient electric motor so it takes a second or two for the barrels to spin up before firing can begin. It has surprisingly low recoil for such a weapon and high accuracy, its high rate of fire combined with relatively powerful rounds makes it a lethal weapon against all targets but it tends to lose a bit of accuracy at long range. It is the heaviest weapon in the game and will slow you down considerably even while sprinting but you can still take cover behind objects (however the sheer bulk of the weapon combined with its wind up time will leave you more exposed than pretty much any other weapon).