If the gas is gradually pumped out, the speed of light inside the tube will increase because the vacuum is created inside the closed cylindrical tube. As we know, light always travels fastest in a vacuum. Light moves slower through denser media because more particles get in its way. Each time the light bumps into a particle of the medium, the light gets absorbed which causes the particles to vibrate a little and then the light gets re-emitted. This process causes a time delay in the light's movement so the more particles there are (the denser the medium), then the more the light will be slowed down.