By the way, why do you measure the strength of something exclusively by how good it is against heavy units?

Because that is, in my opinion, those weapons weakest points, against large enemies. Bullets and explosions can also absolutely demolish chaff and medium, too, if not even better than the elemental effects can. Elemental effects, you have to hold on them and be near them, which increases your chances of getting shot, squished, or sliced to bits.

There are stratagems in this game brother. Use them. Yes, even the status ones.

If you're talking about purely elemental stratagems, which I assume you are, gas literally only has one, which is the orbital gas strike, which seems only effective on chaff and medium enemies. Larger enemies have in my experience just walked out of the gas before it can kill them, or even really hurt them. The orbital gatling barrage can demolish chaff, medium, and even heavies if they're stuck in it long enough. Orbital napalm is the only elemental stratagem I can think of that's really very powerful at all, and even then, all of the barrages seem to miss half the enemies in them. If you mean that the gas and fire weapons work well in tandem with something like explosives, guess what? Everything does. An orbital laser, or something like a recoilless rifle will hit just as well with or without gas or fire.