Dear Guys,
Having the Gattling guns would not have saved Custer's command. (In fact, having walked the battlefield three times, it is easy to see that he could never have even dragged them to the hillside position of his final stand.) The Indians did not charge Custer's command in mass, as depicted in the movies. They thoroughly surrounded it with 2,000 braves, crept up as close as possible using the many ravines, ditches and neighboring hills, and then just poured lead into the little blue mass, non-stop, with lever action rifles, until no soldiers were shooting back. It only took them about 15 minutes. Having two or three guys standing there hand-cranking Gattling guns, all unaimed fire, at barren hillsides and ravines would not have changed much. Indeed, since these guys would have been standing up, they would have been the first to die. (The others were busy shooting their horses in the head, so they would have something to lay down behind. Of course, it didn't help.)
Custer only had one-third of his command with him on that hill, because he had divided his 7th Calvary into three groups, sending them into different directions, to set up the attack. Had he kept them together, all 700+ of them, the outcome of the battle may well have been different.
But, the LARGEST arrogant mistake made by Custer was that before leaving the infantry column two days earlier, General Terry told Custer to take not only his 7th Calvary, but also to take charge of and bring the 5th Cavalry as well. All told, this would have been a huge force of almost 1,500 men. But, Custer refused. He only wanted his 7th Cavalry to have the "glory" of defeating the Sioux, and he didn't want to share the spotlight with the 5th Calvary. (After all, he was planning to run for President.)
Regards, Mannyrock