It writes a gml:MultiGeometry object. Example:
<MultiGeometry gid="c731" srsName="http://www.opengis.net/gml/srs/epsg.xml#4326">
<geometryMember>
<Point gid="P6776">
<coord><X>50.0</X><Y>50.0</Y></coord>
</Point>
</geometryMember>
<geometryMember>
<LineString gid="L21216">
<coord><X>0.0</X><Y>0.0</Y></coord>
<coord><X>0.0</X><Y>50.0</Y></coord>
<coord><X>100.0</X><Y>50.0</Y></coord>
</LineString>
</geometryMember>
<geometryMember>
<Polygon gid="_877789">
<outerBoundaryIs>
<LinearRing>
<coordinates>0.0,0.0 100.0,0.0 50.0,100.0 0.0,0.0</coordinates>
</LinearRing>
</outerBoundaryIs>
</Polygon>
</geometryMember>
</MultiGeometry>