With that many Indians I’m not so sure a Gatling gun would’ve made much a difference.
For 2 reasons:
The Gatling gun was pulled by condemned cavalry mounts and was so heavy and awkward, they would have slowed his advance down. Had Custer taken a Gatling, he would have survived, as he would have been 2 days later getting to the LBH.
Deploying a Gatling gun isn’t like a ma deuce on a tripod that would allow you to strafe a battlefield like we see done in the movies. That is another fault with perception and how many people base reality on what they see in the movies.