I just finished up a shootout between two of the best handheld 3 axis gimbals out there for the GoPro - the PilotFly FunnyGo and the FeiYu Tech G3 Ultra. I've actually bought a couple others, but these are the most polished/functional and can easily be tossed in a box and perform as expected without much fuss. They're both around the same price, in the $300 neighborhood, but the difference they make is night and day. For the outdoor/indoor tests, I hooked them up to my DSLR cage along with another GoPro, so you can see with the same inputs what unstabilized/FunnyGo/G3 looks like. So, hopefully this is helpful to anyone else who has considering picking up either of these (or any) gimbals!
https://www.youtube.com/watch?v=rBzSPdsVfpY
https://www.youtube.com/watch?v=rBzSPdsVfpY