gml:id 

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.

Attribute Information

Used By

Complex Types gml:AbstractGMLType, gml:AbstractGeneralConversionType, gml:AbstractGeneralTransformationType, gml:AbstractMetaDataType, gml:DefinitionBaseType, gml:TemporalDatumBaseType

Source 

<attribute name="id" type="ID">
  <annotation>
    <documentation>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.</documentation>
  </annotation>
</attribute>

Sample