In this paper we present a theory of the gravitational field where this field, represented by a (1, 1)-extensor field h describing a plastic distortion of the Lorentz vacuum (a real substance that lives in a Minkowski spacetime) due to the presence of matter. The field h distorts the Minkowski metric extensor η generating what may be interpreted as an effective Lorentzian metric extensor g = h † ηh and also it permits the introduction of different kinds of parallelism rules on the world manifold, which may be interpreted as distortions of the parallelism structure of Minkowski spacetime and which may have non null curvature and/or torsion and/or non metricity tensors. We thus have different possible effective geometries which may be associated to the gravitational field and thus its description by a Lorentzian geometry is only a possibility, not an imposition from Nature. Moreover, we developed with enough details the theory of multiform functions and multiform functionals that permitted us to successfully write a Lagrangian for h and to obtain its equations of motion, that results equivalent to Einstein field equations of General Relativity (for all those solutions where the manifold M is diffeomorphic to R 4 ). However, in our theory, differently from the case of General Relativity, trustful energy-momentum and angular momentum conservation laws exist. We express also the results of our theory in terms of the gravitational potentials g µ = h † (ϑ µ ) where {ϑ µ } is an orthonormal basis of Minkowski spacetime in order to have results which may be easily expressed with the * Some (odd) misprints and typos have been corrected, some sentences have been changed for better intelligibility and Appendix F has new important remarks which result from discussions that W.A .R. had with A. Lasenby at ICCA10 (Tartu) in August 2014.