Practically, the PG model has many syntactic advantages over the equivalence in expression power RDF+reification. (See excitement over RDF*, which is can be pure syntactic sugar) Syntax is important for usability.
I don't believe that PG in Cypher or GQL will be significantly more expressive than SPARQL 1.1. And in any case are quite different from the tinkerpop model.
I believe it is essential for Neo4J as a growing company that they move beyond their own Cypher to something that is more defined and critically allows them to check a "we are a standard" box on big deals. OpenCypher has solid adoption but lacks coherence between implementations. i.e. same data same query, different result.
Still a more grounded GQL will allow Neo4j competitors to gain on them.