In the GPSBabel GUI, the format is listed as "Flexible and Interoperable Data Transfer (FIT) Activity filefit".
The command line options are as follows:
gpsbabel -i garmin_fit -f <input file name> -o gpx -F <output file name>
Exmaple:
gpsbabel -i garmin_fit -f /Volumes/GARMIN/Garmin/Activity/2014-08-24-09-02-57-Bike.fit -o gpx -F zzz.gpx
There is a way to save/convert to GPX on the Fenix 2 itself:
After you have ended an activity, hold down the menu button, then go to History, and that will show you your recent activities. Select one, then scroll down to and select "Save as Track", and when you connect it to your computer it will be a GPX file.