The benefit of adding new resources is a great idea - would make the game more interesting; more research, etc:
Gas as a weapon, after the attack, when attacker wins the defending fleet recieves 50% of its fleet back, but loses one extra planet - this planet is not captured by the attacker - it is destroyed
Gas as a propellent, perhaps ships with a lower range (but have the `gas` research applied to them) could have its defense modifiers increased by 10-20% because faster ships are moving targets - thus harder to hit. I don`t think you would want to increase the affect to higher range ships
just a quick answer :)