A modified GPI model that accounts for the piezoelectric coupling between the ordered structural elements and the strains εj has been used for studing of effects arising in GPI ferroelectrics under the action of the uniaxial stress p2 and electric fields E1 and E3. The polarization vectors and components of static dielectric permittivity are calcucated in the two-particle cluster approximation for mechanically clamped crystal, and piezoelectric and thermal parameters are also determined. The influence of the simultaneous action of the stress p2 and fields E1 and E3 on the phase transition and physical characteristics of GPI crystal has been studied.