I disagree. Assume it is a closed box reflecting all photons back inwards and perfectly bouncing the gas particles. Now assume here are only two gas particles in the box, a heavy atom and a light one. Assume the box is tall enough that in the presence of gravity there is not enough total energy in the system for either particle to reach the top of the box.

The light one will move faster than the heavier one on average when they come into contact, and in the presence of gravity it will have a higher average height. The same will hold as you add more particles, but there is a curve to it.

It is true hat the atmosphere is more complex, and has things like ozone layer causing temperature inversion due to different absorption characteristics, etc., but the general reasons that H and He are so much more prevalent in the upper layers is largely due to this kind of explanation using gravity.