Access https://connect.garmin.com/ from a web browser on the tablet (force the Desktop version from the browser menu if you get a tablet/mobile specific site), then upload the GPX in Training/Courses/Import.
After that you can go to the Garmin Connect mobile app (if you have it on the tablet or on your phone), select the course under Training/Courses and click “Send to device”.
Alternatively, if you have an USB OTG cable you can just connect the Garmin to the tablet then copy the GPX to the “New Files” folder on the Garmin, it should pick it up from there after you disconnect the cable.