  We follow an old suggestion made by Stueckelberg that there exists an
intimate connection between weak interaction and gravity, symbolized by the
relationship between the Fermi and Newton\rq s constants. We analyze the
hypothesis that the effect of matter upon the metric that represents
gravitational interaction in General Relativity is an effective one. This leads
us to consider gravitation to be the result of the interaction of two neutral
spinorial fields (G-neutrinos) $ \Psi_{g}$ and $ \Omega_{g}$ with all kinds of
matter and energy. We present three examples with only one G-neutrino: two
static and spherically symmetric configurations and a cosmological framework
for an isotropic dynamical universe. Without self-interaction, the associated
effective geometry is precisely the Schwarzschild metric. On the other hand, a
self-interacting G-neutrino generates a new gravitational black-hole.
