  A method is described for calculating the Generalized Parton Distributions
(GPDs) of spin 1/2 hadrons made of composite constituents, in an Impulse
Approximation framework. GPDs are obtained from the convolution between the
light cone non-diagonal momentum distribution of the hadron and the GPD of the
constituent. DIS structure functions and electromagnetic form factors are
consistently recovered with the proposed formalism. Results are presented for
the nucleon and for the $^3$He nucleus. For a nucleon assumed to be made of
composite constituent quarks, the proposed scheme permits to study the
so-called Efremov-Radyushkin-Brodsky-Lepage (ERBL) region, difficult to access
in model calculations. Results are presented for both helicity-independent and
helicity-dependent GPDs. For $^3$He, the calculation has been performed by
evaluating a non-diagonal spectral function within the AV18 interaction. It
turns out that a measurement of GPDs for $^3$He could shed new light on the
short-range nuclear structure at the quark level.
