Garmin tech support, with whom I spoke yesterday, confirms that recent Garmin devices do yield .gpx files, and that the devices are Mac-compatable. I hadn't understood, though, that separate memory needed to added to the unit to allow the .gpx file to be accessed. ..
You don't need to add extra memory for this, at least not in the etrex 30, which has 1.8Gb of internal memory. Unless you fill that with maps, there is ample space