I think the only answer is "they wouldn't be human." It's only reasonable to excrete gas if you either consume an incredibly small amount of matter, or limit yourself to consuming only gasses.
The density of air is 1.225 kg/m^3. This means, at 1/3kg of food per day, we'd have to exude about a quarter cubic meter (~250L) of gas in the ultra-idealistic case. So, the first thing this goes to show is that the concept of bathrooms vanishes entirely, because you must be constantly exuding waste.
However, there's a problem. That's assuming you just turn the food straight into gas. Some foods seem to be easy to do that to (I'm looking at you, evil microbes in my gut making me bloat), but a lot of things are simply not easy to break down into gas. You may need to bring in 4-5 times as much gas to combine with these foodstuffs to turn them into gas, raising the amount of gaseous output dramatically. It also starts to become very energy inefficient. How would you like to consume a morsel of food, only to find that it's going to take more energy to expel it than you got from the food?
Now, perspective. 250L is a lot. Its about equivalent to how much we exhale in 35 minutes. If I have to multiply this by 4-5x, you're starting to see large volumes of gas movement dedicated to this effort. Also, much of that air volume is things like nitrogen, that are really had to use in a chemical process. Consider oxygen as a key reagent we might need, at 21% of the atmosphere. Now we have a need for 5x more gas flow, because we're only using 1/5th of it. Now 3 hours of your daily breathing has to go just to the mass dedicated to gasifying waste, in an ideal case, ignoring the need to burn fuel to do so.
You'd certainly watch what you eat.
Also consider that we have other "excretements" as well, when you start getting down to the dirty side of it. We exude skin cells on a constant basis. You might be able to be sneaky and just piggy-back on that process.
If I were to find a creature that could do this, and ignored the challenge of explaining why it was evolutionary beneficial to limit onesself in such a way, it would be a massive flyer, with a high bypass airflow past its lungs, which would double as a stomach. Just like baleen wales, they'd captrue the little bit of matter they can't get from the air by snaring smaller bugs, and using their bodies. Its waste could be limited to cells shedding off the edges.