The intended use of gml:TopoPoint is to appear within a point feature to express the structural and possibly geometric relationships of this feature to other features via shared node definitions.
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="TopoPoint" type="gml:TopoPointType"><annotation><documentation>The intended use of gml:TopoPoint is to appear within a point feature to express the structural and possibly geometric relationships of this feature to other features via shared node definitions.</documentation></annotation></element>