Gas in Brogue follows realistic laws of diffusion, but only one kind of gas can exist in a tile at one time. This means that if you wish to keep some Caustic gas away you can uncork a Potion of Paralysis or a Potion of Confusion to keep you safe. Gasses disperse especially rapidly over chasm tiles and persist quite awhile within airtight rooms.
Every gas but steam and bloodwort clouds is flammable. Gas will burn and light anything caught in them on fire, but only the explosive gas from bogs and explosive bloats will inflict direct explosive damage. It can often be better to light Caustic gas on fire than to sit in it taking damage.
(Fact check: I don't think this one is true) The gas from popping a pit bloat/uncorking a Potion of Descent (but I think this one is EDIT: confirmed) and from throwing a Potion of Darkness also counts as a gas.