Unless they've changed recently, Garmin Connect doesn't let you import courses to it. However once you've logged that course as an actual activity, you can upload the activity then convert it to a course.
If you like the tedium that would be involved, you could go in and change the tags, headers and other records in the file to look like an activity in .tcx format, but that's more work than I'm even willing to think about. .tcx as well as .gpx files can be changed with a simple text editor.
