Gravitational instability, which is a typical example of hydrodynamic instabilities, occurs due to density difference. When a heavy fluid lies over a light fluid in a constant gravitational field, fluctuations at the interface gradually increase and then macroscopic flows occur. The gravitational instability can be found not only in liquid-liquid interface but also in the gravitational settling of granular materials. However, the gravitational instability of liquid systems and granular systems have been discussed individually in most cases. We quantitatively find a close relationship of the gravitational instability between the physical gel and granular systems. We also find that those behaviors are determined by the thickness of the fluidization region.