Excuse my ignorance as I don't own a gimbal but I always thought they were designed to aid in handling a lens that's too heavy to hand hold. I never thought photographers would try to use them for longish exposures in the danger zone but rather keep shutter speeds high while tracking moving subjects.
My longest lens is a 400/5.6 ED AI and I use it on a 2-way Sinar pan tilt head originally intended for an 8x10" view camera.
Dave Hartman
Jack be gimbal, jack be quick... or so I've thought. 
Yes, the gimbal was developed to aid in holding the heavy lenses, either long or not.
All gimbals work fine, except the cheap ones when they start to have bearing issues, if speed is way up. But then we are facing a dilema, at least for nature photographers. The strong light hours, where speeds can be kept high, are not good in terms of IQ, as you know. Is during the raise and dusk that the changes to achieve a great photo comes alive. But, and not to bring too much noise, the ISO has to be kept within well controlled levels, not same insane ones, that to make thee photo, the speeds went into the red zone. By red zone I mean below the gold standard (speed >= FL). Good technique (NOT the traditional long lens one) should be trained and practice until you can reach success around 1/125s for a 600mm lens. This takes time, dedication and a fluid head. Even some would say that this can be attained on a gimbal head, I would reply, "ok, try the very same doing BIF..."
Now you ask, and why not using VR?? VR is really helpful but it's not a miracle maker. I do use it, and I recommend that you try by yourself and see what you have to change in your technique to master it. From my experience VR is good to use between 1/125s (again this speed..) and 1/500s. New VR modules do well below 1/125s until, let's say 1/60s. But even with VR 1/60s on a long lenses, on a gimbal... good luck! The very same is much easier to achieve with a good fluid head. Bear in mind that sometimes, somehow, sooner than later, you will go below 1/60s to do that shot... when I found myself on this situation I don't touch the lens much, always use a release shooter, and do some bursts. Ahah, the bursts. Wont they induce more trouble than solutions? In fact, if you are shooting landscape please don't do it, but if you are catching wildlife, even if more or less still, try it. You could be amazed...
