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="ApprovalType"><annotation><documentation>Indication of approval of the connected feature or feature collection for a specified purposes by an authority.</documentation></annotation><complexContent><extension base="gml:AbstractGMLType"><sequence><element name="approvedBy" type="womlcore:AuthorityPropertyType" minOccurs="1" maxOccurs="1"/><element name="approvalTime" type="dateTime" minOccurs="1" maxOccurs="1"/><element name="approvedFor" type="womlcore:shortStringType" minOccurs="0" maxOccurs="unbounded"/></sequence></extension></complexContent></complexType>