Are gases affected by gravity?

Of course. Anything with mass is affected by gravity, and gases

certainly have mass.

The mass of all the air above you, given weight by the gravitational

forces that attract it and the Earth toward each other, is the source of

air pressure down here.

In fact! If gravity had no effect on gases, then Earth would have no

atmosphere, Jupiter/Saturn/Uranus/Neptune would be either tiny solid

balls or else nothing at all, and the sun would never have condensed

from the huge cloud of hydrogen.

Yes, all objects are affected by gravity. Newton's law of universal gravity states that any two bodies in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. Therefore we know that gases, and every other atom in the universe, is subject to gravity.

We also know this, as if they weren't affected by gravity, the Earth wouldn't have an atmosphere!