Rogelio,
At the inexpensive end of the market there are not a lot of choices in cameras that produce metadata that can be used for creating a "MISB-like" metadata stream through the multiplexing option. I think well see this capability start to be available in the next couple of years but so far nothing in the hobbyist range.
You hit the nail on the head by suggesting a GPS logger to augment your GoPro collection. I'd go one further and say look at an inexpensive GPS/Compass/IMU unit. For instance, there are open sourced Ardupilot based flight controllers in a 35mm form factor that with a GPS can collect GPS, Roll, Pitch, Yaw, and barometric elevation in flight. The logging function is built in on these flight controllers and you can dump the flight parameters out as a GPX and text file. These are so light you could mount it to the top of a Phantom and collect the data while not actually using the flight controller in place of your integrated DJI controller and would not lose a lot of flight time.
With a little ingenuity you could use a Ardupilot or Pixhawk flight controller that would collect data independent data to use on platforms like the DJI phantom or TBS Discovery that don't have as robust logging capability built in out of the box. In this case, I would rigidly couple the camera to the aircraft frame so parameters could be collected given the offset to the logger. With a two axis gimbal mount for your GoPro ideally you wouldn't have to account for pitch and roll but it never quite seems to work out that way in practice...at least for me.
I'm putting together something similar based on ardupilot that will have a tightly coupled GoPro or Mobius mount on it that will allow me to collect data and video for use on fixed wing models that don't otherwise use a logging flight controller. The goal is to have something 3D printable that can be shared to the community. When that design is complete I'll come back and post a link to it.
DJI does have an add on data collection capability but I'm not familiar with the details and have not used it with the new multiplexer function of the FMV tool.
I hope this helps,
Doug