Most "low carb" beers are brewed with some adjuncts and start at low OG. Less barley = less gluten, but it certainly doesn't mean low gluten or gluten free.
You can lower the carb content of finished beer by
a) brewing low alcohol versions
b) mash for a dry beer - eg mashing very low or long so that most of the carbs ferment into alcohol.
c) add simple sugars that will 100% convert to alcohol
d) brew lagers. Lager yeast eat a wider range of carbs
Of course, those will make your homebrew taste and feel a whole lot more like the commercial versions you are trying to avoid.