Granular gas is a far from equilibrium discrete system. Due to intrinsic energy dissipation, the system will cool down when there is no energy input. In this paper we report our experimental study on this cooling process in a quasi 2-D granular gas system under gravity. By using fast video photography, particles excited by air flow from the bottom of the container were traced during cooling. The cooling process contains two regimes: One of them is when inelastic collisions of particles in the gas phase is dominant. In this regime condensation at the bottom will occur at a rate determined by three factors, namely the constant piling rate α, granular temperature T and vertical mass velocity vg. The second regime is near the end of the cooling, the energy dissipation is mainly contributed from the surface area of the condensation region. This time the piling rate and the dissipation rate both show exponential dependence.