A gatling railgun sounds pretty complicated, but I bet we could manage. The need to have multiple powered magnetic rails would make it pretty darn expensive, though. It would be cheaper to make a gatling cannon powered by just steam or gunpowder, as that way each barrel would only have to be a simple gun barrel, not a powered rail, which requires more precise engineering and more expensive materials.
A rapid fire single barrel railgun would also be possible, but it would quickly overheat and wear down the rails.
I think for rapid fire our best options are either some sort of propellant or a purely magical means of accelerating the projectiles. It's still a bit out of our tech range, given that we've never created rapid fire weapons before, and our existing guns all rely on steam or electricity. It may be a better idea to make a flak cannon, firing bombs with a fuse or proximity sensor, so that it still hits aircraft on near misses.
Or we could go the other direction and invest in air superiority fighters. Vaulters might have an intrinsic advantage in aircraft, though, being a space based civilization.
Ah, but I do agree that a self-propelled railgun with lucky shot built in would suffice for most anti-air applications. Lucky strike makes up for the difficulty in hitting aircraft just as much as rapid fire or fragmentation bombs do.
...That also makes me think we could attach a lucky shot wand directly to a lightning rifle as a "sight". Unly really useful if a mage is holding it, but it should increase the effectiveness of elite snipers.