All three of the components of gunpowder are absolutely essential for human life. Charcoal is basically pure carbon which is kinda required for nigh-everything the human body does. Sulfur is less common in the human body than carbon but a required component of some of the essential proteins we need for our cells to function. As for saltpeter, well there's a very good reason it's present in all our urine.
That said I have no particular problem with gunpowder not working in a specific setting as a base measure. I wish he'd picked something besides "Elements that are absolutely required for a human body to function don't exist" but it's a fantasy, humans may not even be made of atoms in GoT and the wonky physics I can live with as a central conceit. I find the behavior of people somehow less believable, as
@Shipmaster Sane points out, the most noble of Westerosi are basically average by our standards and their average are our evil bastards. It jars me to see it, especially when people claim it's "realistic."