Could life be sustained only by a gas? Humans need food, water and air but can a ET or Alien could just live off gas.
If it were possible, I would think they would be very slow growing, and likely have mouths like a Baleen Whale or the Whale Shark. It would 'filter' large amounts of gas for the elements and dust it needs to survive.
This is assuming that the organism lives in the 'air' at least primarily so that is why it has adapted to this way of living.
I would guess that if one life form made it there would be a pyramid of organisms and of course the larger would feed off the smaller to help concentrate raw materials.
So I guess this isn't really living off 'gas'. It would take a lot of chemical acrobatics to only live off of stuff in a gaseous state, just collecting building materials would be hard. Of course if it is a highly volcanic planet that spews lots of chemicals into the atmosphere might make it a little easier, but once again, it would not likely be surviving on 'gas' alone.
This scenario is highly problematic for organic life forms.
Recall that elements come in three different forms : solid, liquid, and gas. Converting solids to liquids requires heat (thermal energy). Similarly converting liquids to gas requires heat. The hotter the environment, the greater the number of elements that are available as gas. An organic life form which included water in its body could not exist in an environment with a temperature above boiling point.
Take iron, which is essential for human life (and presumably most organic life on earth). Iron has a boiling point of 2862 °C. By comparison, the surface of the planet Mercury is about 470 °C at its maximum.
So any life form capable of sustaining life by consuming only gas would need to be based on a very narrow range of chemical elements. This seriously limits the scope for complexity.
A half-way approach may be this : A very turbulent environment could result in particles of all solid form nutrients to be circulated in the air. But this is kind of bending the rules a bit.
I think your only option would be a non-organic life form - say a computer based life form. All a computer needs to run is energy, and this could be obtained by burning gases (e.g., hydrogen) in the environment.
Assuming it can get traces of minerals from dust, yes.
Consider plants: The primary reaction sequence is combining water and carbon dioxide to make plant material. They normally use liquid water going from the roots to the leaves as a transport medium but a pure gas environment sounds like a zero-g environment, transport will be much easier.
Possibly not on our Earth, for reasons others have mentioned. But if you'll accept any kind of gas, you could have an organism with a biology very much like ours. Just offhand, I can think of two substances that humans can metabolize for energy that are already close to being gases under normal conditions: ethanol and acetone. Make the temperature a little higher, or the air pressure a little lower, and those would predominantly be gas. You could have a respiratory system that uses active transport to efficiently extract them from the air.
How much energy could a vapovore get this way? Humans can exchange about 0.5L of air with a normal breath, which comes to 10,800L per day. If you were breathing pure ethanol at 1 atm and 25C, its density would be 46g/25L (unless my memory of how gases work is way off), so if you could extract 10% of what you inhale, you'd get around 1,987g/day. Humans can get around 7 Calories / g from ethanol, so that's nearly 14,000 Calories per day! Of course, these assumptions are wildly unrealistic, but it shows that even at a much lower efficiency and a lower atmospheric concentration, the idea isn't unthinkable.