Justin, the attached (rather tiny) patch adds support for GML 3.2 AbstractGeometry in xsd-gml2 GMLEncodingUtils. It is required to support app-schema encoding of abstract geometries, and is covered in GeoServer app-schema-test.. The main reason this is needed is that OGC changed _Geometry to AbstractGeometry on going from 3.1 to 3.2, otherwise the existing code would just work. Without this patch, GeoSciML 3.0rc1 geometries are not encoded.
Please review this patch when you have time.