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You can manually upload to the garmin connect website any gpx (and probably other formats too).
If you want things on strava too then you can set the GC Web app to push to strava. I think I have mine set to sync to strava and ridewithgps for maximum opportunities for privacy invasion.
You should pair the power meter with the garmin to get power data into your GPS files.
Manual upload is boring though and you'll probably want to use Bluetooth pretty soon.
If you just want to track distance there is software available for Linux to do this and you don't have to upload anything but it's not quite as swish.
Damn.
Will need to get new Garmins then.