export selected features as gpx for use in qgis action

export_as_gpx.py

layer = QgsProject.instance().mapLayer('[% @layer_id %]')

from qgis.core import Qgis, QgsVectorFileWriter
exportpath="D:/loeschen/test.gpx"
error, error_string = QgsVectorFileWriter.writeAsVectorFormat( layer,
    exportpath, 
    "UTF-8",
    layer.crs(),
    "GPX",  
    onlySelected=True)
    #layerOptions=['...'] )
#print (error[0])

# NoError = 0, ErrDriverNotFound =1, ErrCreateDataSource =2, ErrCreateLayer,
#  ErrAttributeTypeUnsupported, ErrAttributeCreationFailed, ErrProjection, ErrFeatureWriteFailed,
#  ErrInvalidLayer, Canceled

if error == QgsVectorFileWriter.NoError:
    qgis.utils.iface.messageBar().pushMessage("GPX saved", Qgis.Success)
else:
    print('Error: {details}'.format(details=error_string))
    qgis.utils.iface.messageBar().pushMessage('Error creating GPX: {details}'.format(details=error_string), Qgis.Critical)