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
<complexType name="GeodeticCRSType"><annotation><documentation>gml:GeodeticCRS is a coordinate reference system based on a geodetic datum.</documentation></annotation><complexContent><extension base="gml:AbstractCRSType"><sequence><choice><element ref="gml:ellipsoidalCS"/><element ref="gml:cartesianCS"/><element ref="gml:sphericalCS"/></choice><element ref="gml:geodeticDatum"/></sequence></extension></complexContent></complexType>