How can I combine Garmin recorded track data with FitBit heart rate data?
The GPX files produced by GPS devices are just XML so can fairly easily be edited by computer programs (and simple edits like removing sections where your GPS glitched out can even be done “by hand” with a text editor). The XML schema includes fields for all kinds of things, including heart rate.

It seems the FitBit Charge can export data in CSV format, which is also a simple-to-parse text format. Combining these with a script should be fairly easy as long as you know how to program and you use appropriate libraries for, e.g., handling the XML.

Make sure the clocks on your devices are well synchronised. A few seconds’ difference wouldn’t be the end of the world but you wouldn’t want more than that, I’d have thought.