The link below provides a list of supported file formats which can be added/uploaded to ArcOnline. Can GPS files be uploaded to ArcOnline? Can a GPS Exchange file (.gpx) be uploaded to ArcOnline inside a .zip archive?
Link: http://doc.arcgis.com/en/arcgis-online/share-maps/supported-items.htm
Michael,
You should be able to add gpx files according to the help, no need to place inside a zip file:
CSV, TXT, and GPX files—Help | ArcGIS
Regards
Anthony