gpsbabel -w -r -t -i gpx -f /home/USER/PATH-TO-FILE -o garmin -F /dev/ttyUSB0You can enter this line also in gebabbel (a GUI for gpsbabel) in "Edit command" to use it from there.
It's dirty (look where your gpsr is connected can be /dev/ttyUSB1 etc. ) works if you get the error: "Claimed interface failed: Device or resource busy"
So for sending the file tour.gpx in the folder Documents in your home directory and your linux-username is mandark e.g. enter gpsbabel -w -r -t -i gpx -f /home/mandark/Documents/tour.gpx -o garmin -F /dev/ttyUSB0