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Summary form only given. A quasi-phase-matched backward second harmonic generator in a waveguide can be viewed as a one-dimensional nonlinear photonic band gap crystal. At low powers the system is transparent to the fundamental frequency (FF), whereas at high powers it exhibits a large reflectivity due to up-conversion to a counter-propagating second-harmonic wave. We predict that, in the presence of an additional Kerr response, new fully transmitting states are present. These are associated to the formation of two-color electromagnetic energy localizations in the form of envelope parametric solitary waves (nonlinear-gap solitons). The existence of such states is confirmed both by the numerical and analytical investigations.