A gml:PrimeMeridian defines the origin from which longitude values are determined. The default value for the prime meridian gml:identifier value is "Greenwich".
The attribute gml:id supports provision of a handle for the XML element representing a GML Object. Its use is mandatory for all GML objects. It is of XML type ID, so is constrained to be unique in the XML document within which it occurs.
Source
<element name="PrimeMeridian" type="gml:PrimeMeridianType" substitutionGroup="gml:Definition"><annotation><documentation>A gml:PrimeMeridian defines the origin from which longitude values are determined. The default value for the prime meridian gml:identifier value is "Greenwich".</documentation></annotation></element>