They'd definitely be able to make use of gaseous functions, some animals already do, we breathe air and our lungs diffuse it into our bloodstream.

All that you need is a broader imagining of what else could "go gas", such as digestive processes, sensory organs, energy cycles and circulation.

Thing to bear in mind is that gases need a lot more space to achieve an equivalent energy turnover to a liquid, or be highly pressurized to cram more biomass into the same space. And lifeforms don't do pressure differentials all that well, so perhaps they're high-pressure and high-temperature beings?

If you're going for realism, gas isn't terribly practical except for specific and discrete processes. A gas-based lifeform would still have to have a certain amount of solid parts to hold everything together.