I used to have a 205, but have not used one for a while so this from memory...
This
site is essentially a front end to GPSBabel which should help you convert the file.
The command to transfer files should look something like this, assuming the file is called foo.gpx:
gpsbabel -w -r -i gpx -f foo.gpx -o garmin -F usb:
The 305 can do routes and courses. Courses are akin to following a breadcrumb trail whilst routes are waypoint to waypoint. Depends which you want, as to how gpsbabel should process waypoint information.