It writes a gml:MultiPolygonType object. Example:
<MultiPolygon gid="c731" srsName="http://www.opengis.net/gml/srs/epsg.xml#4326">
<polygonMember>
<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>
</polygonMember>
<polygonMember>
<Polygon gid="_877790">
<outerBoundaryIs>
<LinearRing>
<coordinates>0.0,0.0 100.0,0.0 50.0,100.0 0.0,0.0</coordinates>
</LinearRing>
</outerBoundaryIs>
</Polygon>
</polygonMember>
</MultiPolygon>