Light is always light and has always the same speed but its path in a gas is less straightforward than in a vacuum because of the interactions with atoms. It takes longer to get through. Its speed as we can measure it is c divided by the refractive index of the gas, if I'm not wrong.
Same thing for light in a liquid or in a solid. Example: speeds of radio waves in networking cables https://en.wikipedia.org/wiki/Velocity_factor#Typical_veloci...