OK. In your case, you can just use the GeometryPropertySetter with an appropriate Geometry XQuery in order to add the "gml_id" trait to the Polygon part.
GeometryPropertySetter: Geometry XQuery | Test Clauses
Part to TestLeft ValueOperatorRight ValueThis partGeometry Type=IFMEPolygon
Assuming that the input surface feature has an attribute called "gml_id" that stores the Polygon ID value (e.g. "id_19_205_polygon"), the parameters setting looks like this.

If the Polygon part always has the geometry name "exterior" (and other parts don't have the same name), this test clause would also be available.
Part to TestLeft ValueOperatorRight ValueThis partGeometry Name=exterior
The most suitable Geometry XQuery setting could be vary depending on the conditions and the requirements.
Hope this helps.