A very disappointing response from ESRI, as the lack of support for this tool in Portal is not documented. I wouldn't have gone through all the trouble to deploy it, then spend months troubleshooting it, and hours on the phone explaining the problem. Why aren't GeoJSON and GPX supported in Portal? Isn't Portal the same platform as AGOL?
The status of Esri BUG-000104018 - When Web AppBuilder for ArcGIS (Developer Edition) is configured with Portal for ArcGIS, adding a .gpx file using the Add Data widget fails. - that you have been associated with has been changed to Not in Current Product Plan.
Public Explanation: GeoJSON and GPX files are not supported in Portal for ArcGIS backend.