This paper presents a fast SPH (Smoothed Particle Hydro-dynamics) simulation approach for gaseous fluids. Unlike previous SPH gas simulators, which solve the transparent air flow in a fixed simulation domain, the proposed approach directly solves the visible gas without involving the transparent air. By compensating the density and force calculation for the visible gas particles, we completely avoid the need of computational cost on ambient air particles in previous approaches. This allows the computational resources to be exclusively focused on the visible gas, leading to significant performance improvement of SPH gas simulation. The proposed approach is at least 10 times faster than the standard SPH gas simulation strategy, and is able to reduce the total particle number by 25-400 times in large open scenes. The proposed approach also enables fast SPH simulation of complex scenes involving liquid-gas transition, such as boiling and evaporation. A particle splitting and merging scheme is proposed to handle the degraded resolution in liquid-gas phase transition. Various examples are provided to demonstrate the effectiveness and efficiency of the proposed approach.