Gas is a material not affected by gravity. The Gas material and Steam contain Light to make it pretend to be lighter than the other materials. Gas is one of the only default material in OE-Cake with transparent particles in Dots viewing mode, the other being Snow. Gas added to a mixture makes that material float.
Pure Gas, made by pressing esc + G + esc, is neutrally buoyant and is the perfect material for creating an atmosphere. Gas can be used to fire cannon balls and to make shockwaves and to provide drag or pressure on an object. Gas can create vortices and pretend convection. Pure Gas can "dissolve" in Water because it doesn't have Light to force it out. Water permeated with Gas will slowly evaporate it out because Water preferentially moves down whereas Gas is not affected by gravity.
Gas in OE-Cake isn't quite realistic because it doesn't "fill volumes" like it does in real life. The Gas just sits there until it is moved, and the particles in standing volumes never move to create Brownian motion. Using the resistanceFlag variable in the Parameters, you can create a special type of self-inflating Gas that exhibits Brownian motion and static pressure space-filling properties. Setting resistanceCoefficient to -0.00001 gives all particles in the game a tiny amount of force. This tiny force is not enough to significantly change every material in the game. Bulk materials rarely move enough to be affected by the resistance variable because resistance has no effect on stationary particles. However this force is *just* enough to keep Gas particles always gently moving, causing them to create a plasma-like soup as they bounce off each other. This motion creates momentum and pressure in the Gas, allowing it to spread around to fill an entire shape evenly and provide a gentle inflating force. This technique allows you to have a more realistic Gas atmosphere using less particles, because you do not need to completely pack the entire area with Gas.