I like the way that OSM presents relation/way/area; for example:
I would like to generate a map (likely using gpsvisualizer) that combines these two relations with other way/area/node and also some of my own data (GPX format).
When I download XML from OSM using link in above relation page; it only contains a list of ways. How can I obtain a file for the above relations that contains coordinates?
try overpassTurbo with the IDs of the relations, which allows exporting in gpx, kml, geojson [Export button]
If you potentially want to do this for any relation (or way, or whatever) then you could try throwing your OSM data into a database with e.g. osm2pgsql, and fetching data from that follow the switch2osm guide and use a very limited stylesheet).
This is probably some way from what you are trying to do, but I thought I’d mention it just in case.