$ sudo npm install gopro-utils -g$ gpmd-extract GX000123.MP4 GX000123.JSON
The JSON file's contents corresponds to the output of the
gopro-telemetry
library, see here for
more details.
This CLI app is a very thin wrapper around Juan Irache's
great GoPro libraries
(gpmf-extract,
gopro-telemtry),
he is the one deserving of all the praise!