Points and lines only
GPX holds waypoints, routes and tracks, and mapverter writes waypoints and tracks. Point features become waypoints, with one waypoint for every point of a multipoint feature. Line features become tracks, and a multipart line becomes one track with several segments. A polygon Shapefile cannot be stored in GPX, so the download is blocked and the reason is shown.
Which attributes survive
GPX has room for a name and a description, not a full attribute table. mapverter keeps the value of a field named exactly name as the waypoint or track name, and a field named desc as its description. Every other field is dropped, and the conversion report counts how many.
Coordinates
GPX always uses WGS 84 longitude and latitude, so the target is locked. mapverter reads the .prj by EPSG code or by its parameters and converts every coordinate. If the Shapefile has no .prj, coordinates within longitude and latitude ranges are treated as WGS 84. Otherwise you choose the source system from the offered candidates, by clicking your area on the map to try the local UTM zone, or by searching for an EPSG code or name.