Confined longitudinal coherent acoustic phonon modes are excited and detected in a sub-μm-thick free-standing cubic GaN membrane by femtosecond pump-probe spectroscopy. After fs laser excitation, Brillouin oscillation and thickness oscillation with frequencies up to 100 GHz are observed and studied in the time domain. We found an initial expansion of the membrane upon optical excitation at 400 nm. Our experimental results confirmed earlier existing theoretical predictions and experimental observations that the hydrostatic deformation potential of cubic GaN is positive.