When I plug my Garmin GPS into my laptop without the Garmin software running, the GPS shows up as a drive. On that drive is a directory named GPX. I copy the files I've downloaded from HD into the GPX directory - it has to be in that directory to be able to see them in the next step of the process.
Once the file is in the appropriate directory I unplug the GPS from the laptop and turn it on. At this point if I go to the "Routes" icon under "Where To" the routes will not show. I have to go into "Settings" and then "Routes," select a route and then I get a number of options one of which is to "Import" the route. Once imported the routes show up under Where To - Routes.
It's not the most intuitive process but it's not difficult.
Rever lets you download GPX files, which I copied to the internal GPX folder of my Garmin AND to the GPX folder on the external SD card, but they don't show up until days later. Again, older Garmin with some apparent issues.