He did deliberately leave the gatling guns behind. They also left their sabers behind for the same reason - they were trying to move quickly.
Been told all my life that Custer and his men could have cut down scores of Indians with their sabers had they retained them and maybe even broken out and escaped.
But the Indians had rifles. And arrows have more range than sabers. They massed their forces. A fatal combination for outnumbered U.S. cavalry.
But the Indians’ victory was only temporary. Until casinos.