In this paper, we propose a physics based model for the gate capacitance, surface potential and channel charge for III-V symmetric DGFETs. This model comprehensively accounts for different terms that contribute to the gate capacitance such as insulator, centroid and quantum capacitance. It considers 1D confinement of electron, wave function penetration into the insulator and Fermi-Dirac statistics. It contains only one approximation that the shape of the wave function is independent of applied gate voltage and this approximation is taken care by adding perturbation term to the energy of sub-bands. We show that the model matches very well with data obtained from the 1D Poisson-Schrödinger solver for different channel thicknesses and materials.