How fast is the speed of Light in a gas?

Answer 1

The only general answer that's true in all cases is: Slower than in vacuum.

The exact speed depends on which gas, and the density of the gas, which

in turn depends on its temperature and pressure.

Answer 2

The speed of light in a gas is slightly slower than its speed in a vacuum. This is because light interacts with the particles in the gas, which can slow it down. The exact speed of light in a specific gas will depend on the composition and density of the gas.