Difficult question. When I look at their firing rates, it's 160 rpm vs. 1220 rpm and it makes me think "no comparison, Vulcans/Pivads all the way. But then real damage values, which are arguably just as important as rof, are not shown to us in the in-game unit stats. The Gepard, having 35mm guns, probably does twice as much damage, but it doesn't make up for it as Vulcans seems to kill much faster still. Also, the base model FLAKPANZER M42A1 shoots at 288 rpm, and has 40mm guns, so it probably has higher DPS than both the Gepard variants.
However, Gepards have 2.275k range as opposed to Vulcan's 1.75k range against flying targets (range against ground targets seems to be 1.5k for both), this combined with the fact that the Gerpard A1 has got a stabilizer, makes it a possible counter against helicopters shooting their rocket pods from maximum range whereas Vulcans can't too much about them. But then, Gepards, especially at their maximum range, damage helicopters quite slowly mainly due to their low rof. So when you're countering these helis with rocket pads with them, it's almost an even fight (mind you, the Gepard A1 costs a whooping 50 pts) and you don't want and even fight when you can almost instantly and reliably crush those Helis usings sams. At 50 pts, it's not a very good value when you consider what the 2K22 Tunguska does for 65 pts.
Yea, Vulcans/Pivads are teh winrar from my point of view as well. FlakPanzers are a good choice if you're planning on not using any sams at all, for some reason.