Gas containers are used in Forerunner mining facilities to move the volatile gas mazes from one location to another within the facility. They can detonate if exposed to weapons fire or an explosion in their immediate vicinity. The gas inside these containers is possibly Methane, since it appears to be similar in color to the gas contained in the tanks which the Unggoy have on their backs.
In Halo 2, they can be used to kill Heretic forces on the level The Arbiter and The Oracle. They are seen on conveyor belts most of the time and also spawn in the multiplayer map Colossus. These containers are better to use when there are groups of enemies to damage their energy shields for an easier kill.