Beg to differ. Any kind of horse-drawn, wheel-mounted and crew-served weapon would have been perfectly useless to Custer. That becomes clear to you if you've traced the route of Custer's command along the ravine-studded ridge. The only conceivable way they could have saved him would be if they had delayed his advance long enough for him to realize what he was up against and pull back in time to establish a defensible position somewhere (as Reno/Benteen were able to).
Artillery in the Indian wars was ever only usable when either the Indian targets were stationary, as in the siege of the Lava Beds during the Modoc War, or when the troops were defending an established position, like Kit Carson at Adobe Walls in 1864. In 1877 at Big Hole, in a battle of about evenly matched numbers, the troops botched an attack on a Nez Perce position, tried to use a howitzer on a fluid battlefield and promptly lost the gun to the enemy after firing a few ineffectual shots.
Considering how fast the battle moved along the Little Bighorn, and how unstructured the enemy (and quite possibly Custer's command toward the end) operated, any speculation that deploying any artillery, including the much-overrated Gatlings, would have been possible, and could have substantially altered the outcome, seems a stretch.
Maybe if Custer had been able to sneak up and hit the village with a surprise bombardment from a well-established hilltop position, the Indians would have run. But that's even more if a what-if stretch.
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