Since August 1st, RunGap has been unable to import workouts from Garmin Connect due to an issue with the Garmin Connect service. According to Garmin this is a known issue that their engineers are looking into.
You can reproduce the issue by logging in to the Garmin Connect website, opening a workout and try to export it in "original" format. At the time of writing, this results in an error "400 Bad request" error message.
Unfortunately we will have to wait for Garmin to fix it.
In the mean time you can copy your FIT files directly off your device and import them to RunGap via iCloud Drive, DropBox etc.
Thanks for your patience.
Fixed by Garmin on August 5th.