You can go to the upload page. Near the bottom of the page click Manual Upload, then click Choose File. When the Open dialog appears, locate the gpx file and click Open, then click Upload Activities From File.
Note that Garmin Connect may not accept your gpx file at all. It seems to have some quirks. I've found workarounds for the few issues I've seen, but I'm sure that there are others. If you run into trouble, upload your gpx file here and I'll take a look at it.
