See, gas is a form of matter, right? Yes. Matter or in this case, volume, takes up space so no matter what, since it has partices, and atoms, it's always going to take up space. Take water vapor as an example, that's gas, right? Yep. It forms when liquid is heated. You see it when your boiling water, or making tea the old fashion way.
Hope this helps! Virtual Smiles...
~Mqddie S.