There is some helpful code at http://www.espruino.com/Bangle.js2+Technical#gps that might help? It at least calculates the CRC
@Gordon started
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
There is some helpful code at http://www.espruino.com/Bangle.js2+Technical#gps that might help? It at least calculates the CRC