All,
We've identified a bug on May9th related to this topic,
> 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.
We are currently investigating the problem. If you could please open an incident with Esri Tech Support, that may help us to track down and resolve the issue.
Hope this helps,