I have flown an FPV quadcopter with a GoPro and used an Immersion 5.8 GHz transmitter for the video signal. The big disadvantage of the system was the absence of a gimbal, so there was a jerky video with lots of jello.
I understand the CGO3+ and ST16 have a unique video signal which means that the signal from common video transmitters such as the Immersion cannot be picked up by the ST16. If you intend using the ST16 as your video receiver, I think your best option is to use the transmitter from a CGO3+. Perhaps there are cheap crashed CGO3+ cameras on Ebay? However, I do not know if the GoPro can be linked to a CGO3+ as I have not read of anybody trying it.