@fpvsteve did a couple of write ups on the subject, he had full camera control iirc. Here is one thread of many here on the forum, search is your friend...
Thanks to @just_bruce for helping me out with a trashed gimbal to experiment with, I am on my way to fit the newer Gopro models into the 3DR gimbal. Still some work to do with the controls and video feed, but certainly doable.