I made one from a couple of bolts and a piece of delrin, and then I made another where I added some skateboard wheel bearings. The GoPro I have has an inverted video mode, so I just let the camera hang upside down.

Level is maintained by the camera's own weight. Damping is provided by friction (bolt to delrin in the first case, by adjusting bolting force on the bearings in the second).

I hope that helps,
Eric