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.
<element name="Point" type="gml:PointType" substitutionGroup="gml:AbstractGeometricPrimitive"><annotation><documentation>A Point is defined by a single coordinate tuple. The direct position of a point is specified by the pos element which is of type DirectPositionType.</documentation></annotation></element>