also he did not want to carry two types of ammo...the older 50-70 Ammo for the Gatling's and the newer 45-70 ammo for the Model 1873 Trapdoor Springfield Rifles/Carbines. Bringing the Gatlings could have made a difference. Also the Army brass didnt like the idea of wasting ammo in repeating rifles so even though the Henry and Spencer repeating rifles were used by Union forces in the Civil War, they were put out of business after the war. For long range high power shooting the Trapdoor Springfield was the superior weapon, but for fighting the close-in guerrilla tactics of the Indians, it was way to slow. the Indians put their repeaters to good use.
Gatling guns were far too heavy for a cavalry unit. Impossible to take them along.