So a wizard in Gaseous Form gets based by an air elemental in Whirlwind Form. What exactly happens in the next couple of rounds?
Don't confuse reality with rules, IMO.
The Whirlwind needs to be one or more sizes larger than the wizard to do anything to him. Gaseous Form does not change this.
That said, if somebody wanted to try it out in a home game, I would totally allow it. Against gaseous form, and against incorporeals, for that matter.