  The gravitational coupling parameter G is determined by a non riemannian
curvature scalar of a background substratum. This substratum represents an
inertial solution to the nonlinear equations of a geometric unified theory and
provides a limit solution to general solutions in the theory. When a solution
approaches the substratum solution as a limit we physically obtain a newtonian
limit of the gravitational sector of the unified theory. The curvature scalar
is determined by the interaction mass-energy content of the solution. In the
limit the equation reduces to Poisson equation and the curvature scalar reduces
to the constant substratum curvature scalar which determines Newton
gravitational constant G. Outside the limit the curvature parameter replaces
the newtonian constant G as the coupling parameter of Einstein equation of
gravitation. This new parameter may be approximately constant in certain
physical conditions but, in general should be considered a variable which
depends on the matter and energy on space-time. This effect and the geometric
structure of matter and fields may be interpreted as dark matter effects. The
substratum also determines a constant mass parameter for field excitations
which obey the Dirac equation and behave as particles. Physically we consider
that the substratum provides two related mass scales: the gravitational
constant G characterizes a macroscopic mass scale and a fundamental particle
mass m characterizes a microscopic mass scale. The substratum geometrically
represents the physical concept of inertial system.
