I did some testing with Gas damage and Electricity damage (against multiple different Faction types) and found that Electricity basically outperforms Gas on all accounts. Because the Gas build has Bane mods that compensate for Faction-based resistances, I think that the disparity in damage is because Gas' proc formula for damage over time doesn't include Heat or Toxin; but it used to include Toxin. The procs also cap out at ten, which no other DoT proc in the game does.
I'm really hoping that this formula is an error and that DE didn't arbitrarily make Gas the worst Elemental damage type