How to extract GPS information from a video stream

Question

I just got a Garmin Virb Elite VCR and recorded some videos with it, and also confirmed that my own Garmins player can display GPS information on a map.

This VCR stores GPS data in a separate .gpx file, which can be easily opened with text editors (xml).

I am sure that gps data is also written to the video stream as a riff with consecutive identical data bytes (piece).

I'm looking for some information to help me extract this data using C #.