Does gravity affect the trajectory of gas particles? We're studying the kinetic theory of gases in school, and one of the points that was brought up was that: "Gases consist of particles in constant, random motion." How is it possible for gas particles to move in straight lines, entirely randomly? Doesn't gravity affect them, or is their mass so small that its effect on their movement is negligible?
Yes gravity pulls on gas molecules. That is why the atmosphere doesn't just float off into space. 

The gist is that the time between collisions is very short in the lower atmosphere, and the distances very short. The mean free path at atmospheric pressure is only about 70 nanometers. So the assumption is that gas particles travel in a straight line between collisions.