Did anyone find the answer or are you still looking?
An open hardware site or something? It might be fully 3-d printable although sometimes that's heavier than just flat parts.
For firmware and electronic hardware, I know these 2:
"Martinez", using an AVR and controls 2 axes. You can get compatible hardware at eg. RCTimer. I have written a new firmware, still in test, which i think performs better.
EvvBGC uses an ARM CPU and supports 3 axes. The current state of the project is a mess to find out about though - unless you like to read through 200 pages of RCGroups.
For gimbal hardware: I have seen some talk but not much concrete. Anyway small gimbals can be made just fine out of small aluminium angles, bolted directly on the motors. If not quite balanced, 10 minutes and 0.50 dollars later you have the next try.