We investigate droplet impact on a solid substrate in order to understand the influence of the gas in the splashing dynamics. We use numerical simulations where both the liquid and the gas phases are considered incompressible in order to focus on the gas inertial and viscous contributions. We first confirm that the dominant gas effect on the dynamics is due to its viscosity through the cushioning of the gas layer beneath the droplet. We then describe an additional inertial effect that is directly related to the gas density. The two different splashing mechanisms initially suggested theoretically are observed numerically, depending on whether a jet is created before or after the impacting droplet wets the substrate. Finally, we provide a phase diagram of the drop impact outputs as the gas viscosity and density vary, emphasizing the dominant effect of the gas viscosity with a small correction due to the gas density. Our results also suggest that gas inertia influences the splashing formation through a Kelvin–Helmholtz-like instability of the surface of the impacting droplet, in agreement with former theoretical works.