I purchased a SJCAM 4000... it's almost exact size as the gopro, works wonderfully. I purchased from Amazon.ca for approx $100... beware though, there are knock-offs of this camera on amazon and ebay... they say sjcam in the ad, but it's not a real sjcam and won't work with the app.... the camera should actually say 'sjcam' on the front of it.. if it doesn't then beware. Kind of funny cause the sjcam is a knock-off of the gopro... so these other cameras are a knock-off of a knock-off?? Anyway, the SJCAM is great and works well with any gimbal that advertises to fit gopro, as the dimensions are very similar.
Good luck.