Is there a gas with metallic luster?
Good question... would a plasma qualify? Metallic luster is due to a sea of electrons, free to move and to reflect (well, re-radiate) incident radiation. A plasma can be created in a gas, such as the interstellar medium, or in Earth's atmosphere. The Kennelly-Heaviside layer is such a "shiny", reflective layer, causing skip signal propagation to extend the range of radio communication. So if you saw in radio wavelenghts, rather than the visual spectrum,  plasma is shiny!

Another possibility would be a Bose-Einstein condensate... but the act of illuminating it tends to destroy it.