Is it legal to own a Gatling gun?

Depends on where you live, and how the Gatling Gun is operated. In MOST of the US, a hand cranked Gatling Gun is legal to own, if you can meet the legal requirements to own an ordinary gun. However, the last one I saw for sale was around $15,000.

If the Gatling Gun is operated from a power source (think Jesse Ventura and the movie Predator), then it is regulated as a machinegun. That requires special registration and payment of fees.