The SG117 is much more akin to a gigantic revolver than a gatling gun... except it would have shot off seven shells each shot!

My personal favorite is the MK112, a 55mm
automatic cannon:
http://www.luft46.com/images/mk112-2.gifIf you look at the design, it basically appears to be a scaled-up MK108. This was not employed operationally, and thank God it wasn't! The Mk108 could supposedly destroy a bomber in 5-8 hits, so this thing would be rape incarnate! Of course, as this would, like the MK108, assumably be a low-velocity cannon, the pilot shooting at the bomber formation would have to get quite close as well, putting him in considerable danger...
Another neat one that saw service trials was a 21cm rocket mortar projector assembly derived from a rocket artillery battery. This was to be fitted to the Me 410 fighter in the fighter's weapons bay, where six internally-mounted tubes would be able to revolve into battery and successively launch a warhead - potentially at a
very high rate of fire. It wasn't really very sucessful (and the 21cm rocket mortar wasn't really accurate to begin with) and development was stopped. There's an article about this at the bottom - if I can find the actual assembly on the '410, I'll post it:
http://en.wikipedia.org/wiki/21_cm_Nebelwerfer_42