We briefly discuss the possibility to describe with a formalism, analogous to the Bragg law and the transfer matrix method used for photonic crystals, the behaviour of the kinetic energy of an object travelling through a one-dimensional (1D) modulation of the gravitational interaction, i.e. a 1D gravitational crystal. We speculate that certain ranges of the kinetic energy of an object with mass m and speed v cannot travel through the crystal, giving rise to a gravitational gap.