it seems that the divergence of this superpotential yields the conserved current due to the Bianchi identities. The divergence of this superpotential is then automatically zero.
You should check "Gravity and Strings" by Ortin, section 2.2 for some extra comments on this superpotential :)
So in the context of GR I would say that gct-invariance implies conservation of the energy momentum tensor, and this energy momentum tensor can be written as the divergence of the superpotential due to the Bianchi identities.