Script to convert Garmin FIT files to Google Maps JS API heatmaps javascript include.
See https://www.musings.ch/2020/02/22/all-garmin-tracks-on-a-map/ for how to get all Garmin activities, then use this script to create a javascript file to include in your Google Maps JS API heatmap.
See https://www.musings.ch/2020/02/26/garmin-tracks-heatmap/ for how to generate a heatmap of your Garmin tracks using https://jsfiddle.net/ (or simply running it in your browser).