I attach my own web site link which I use to convert Gpx files to a viable route plan for my own use, using ANY Gpx file, with sensible limits.
It accesses Google Elevation if that is unavailable and NOAA data for magnetic variation and human walk rates built on years of experience with a club walking factor.
It clearly deals with Garmin files which don't have elevation information by adding that info ..so please try that and it automatically adds that info to produce a pdf formatted downloadable Route plan for me.
Try it by loading a Gpx file, or you are free to and evaluate it, try with any < 40 waypoint route!http://gpxelements.azurewebsites.net/
May be interesting