You can create a Strava route from a GPX file by selecting Dashboard from the top navigation menu on the Strava website then My Routes > Create New Route. Click the upload button highlighted below and choose the GPX file you would like to use to create a route. Once the file has been uploaded, you can adjust the waypoints on the map or edit your route preferences as necessary. Select Save once you've finished. More information about Strava routes can be found here.