I don't understand this. What is the massless scalar field you're talking abut? Can you describe it?
It seems to be called the dilaton or radion, and I can't describe it physically because it has never been observed (doesn't exist?). It seems to show up in all string field theories, and a great deal of effort has been put into explaining why we don't observe it (under the assumption that string theory is right).
Personally, I expect the GUT (in Einstein's sense--*all* of the forces, not all but gravity) will be purely algebraic, along the lines of A. Connes reconstruction of the standard model from non-commutative geometry and the Barrett-Crane model for QG, with the appearance that we live in a Ricci-flat smooth Minkowskian 4-manifold being an artifact of the theory.
I myself am a pure mathematician who works in areas influenced by quantum physics (often collaborating with Crane) so I don't think I can do justice to the explanation for a physicist.