As long as everyone is in a frenzy over realism, I say that gas giant troops should get a 1000% bonus when attacking non-gas giant troops. Because the gravity on a gas giant has to be at least 10x that on the other worlds, they should be able to kick butt against their wimpier 1 standard G colonists/troops.
(Never mind that troops are more like cargo items stored in the planet that have nothing to do with colonists and are therefore atmosphere non-specific...)