MPSuperShape can import and export GML files that use MPSuperShape's own GML schema. GML ('Geography Markup Language') is an open standard based on XML that is intended for geographical information. The GML standard is controlled by the Open Geospatial Consortium. MPSuperShape's GML schema uses the namespace prefix of mpss. The XML schema is available for download here:
See the Importing Shapes section for an example output file produced by MPSuperShape.