Gas lasers all have in common the same pump source: electricity. The gaseous species enter the excited state either directly, by collision with electrons, or indirectly, by collision with other gases, themselves electrically excited.
Gas lasers cover the whole optical spectrum, from the ultraviolet to the far infrared. However, the spectrum is not continuously covered: gas lasers emit very narrow spectral lines. The most common gas lasers (from the UV to the far IR) include:
excimer lasers (ArF:193 nm, KrF:249 nm, XeCl:308 nm)
argon-ion lasers (blue and green wavelengths)
helium-neon lasers (the neon is used for the laser effect) 632.8 nm, 543.3 nm, 1.15 μm, 3.39 μm
CO2 lasers: a large number of wavelengths around 9.6 μm and 10.6 μm.
Only CO2 lasers are really efficient (15 to 20%). They are used in industry for processing materials. The efficiency of the others is mostly less than 1%. Gas lasers are often bulky and need a great deal of water-cooling (almost all the energy provided by the pump is lost as heat). Even though those operating in the visible (Argon, Helium, Neon) are tending to be replaced by solid state lasers, excimer lasers and CO2 lasers are still very frequently used (for the treatment of materials in the broadest sense).