Where i can a find a widget to load GPX on web app with webappbuilder for developer with Arcgis online ?
In the next update to Web AppBuilder embedded in ArcGIS Online, we're enhancing the Add Data widget to support adding local data files, including GPX data. The update is planned for mid-Dec. This might meet your needs.
An updated release of Web AppBuilder Developer Edition will follow 5-6 weeks after the ArcGIS Online update.
Hope this helps,
FYI: GPS Exchange Format - Wikipedia
Retrieving data ...