Pultruded glass fiber-reinforced polymer (GFRP) bridge decks distribute punctual vehicular loads to the underlying superstructure and can also act as the upper chord of hybrid main girders. The deck’s structural performance in both cases is influenced by its transverse behavior. The static bending behavior in the transverse-to-pultrusion direction of two GFRP bridge deck systems with trapezoidal and triangular cell cross-sectional geometry was experimentally studied. The deck with trapezoidal cell core exhibited significantly lower apparent bending stiffness and in-plane shear stiffness than the deck with triangular cell cross-section. The two studied decks showed different load-displacement behaviors and failure modes in their transverse direction. Load transfer mechanisms were also found to differ depending on the cross-sectional geometry. An existing analytical method for composite girders with flexible shear connection was used to obtain the in-plane shear stiffness of both decks in their transverse direction using the experimental deflection results.