GFRP sections with filler concrete material form promising structural components for structures; therefore, the structural performance of them has been investigated with increasing popularity. However, the performance of these composites degrades when fully composite action cannot be developed at the interface in where the literature hosts limited knowledge. Different techniques, such as abraded and sand-bonded surface treatments, were investigated experimentally to improve the bond-slip behavior between GFRP and concrete; however, there is a need to define shear mechanism at the interface of the numerical models. In this study, first, the average frictional bearing strengths were extracted for the treated and untreated inner surfaces using experimental results; then, the coulomb friction model was utilized to transfer the shear stresses between two dissimilar materials. Numerical models were verified by the experimental results, and different parametric studies were investigated by varying the amount and shape of GFRP in the cross section, compressive strength of concrete including the non-linear material behavior and interface frictional contact models. The findings showed that the interface strength can improve the flexural capacity of the concrete-filled GFRP beams by about 15.4%. Square GFRP box sections can be suggested for the construction of hybrid beams instead of rectangular sections, whereas the 10% areal ratio in a square cross section reached 103% load capacity improvement. The increased nominal compressive strength of concrete in hybrid beams can increase the hollow GFRP beams’ nominal load capacities and elastic stiffness of the hybrid beams in between; however, the relative gain is reduced due to increased compressive strength of concrete.