Tiny tools can be a great help.
I was searching quite a while until i found a way how i could convert a GPX-file into a IGC-Datei on a Mac. As there are numerous tools in the Windows-world, there are only few of such little helpers for Apple users.
Stein Sorensen’s GpsDump is a small tool for Mac users (macOS High Sierra) which I use to convert GPX-files into IGC-files easily.
A GPX-file can be read directly from my GPS-Handheld GPSmap 62s and then be converted into IGC-, WPT-, CUP-, KML- or GPX-file formats. Devices from Bräuniger, Flytec, Flymaster and Garmin are supported.
GpsDump is for free and can be downloaded here: http://www.gpsdump.no/body_mac.htm