I never understood why when gas chambers were in use, it wasn't something simple, cheap and basically painless, like carbon monoxide? It's still used for euthanizing small animals, and considered humane.
But, no, US gas chambers mostly were about putting potassium cyanide pellets in a jar under the condemned and then pumping in
sulfuric acid over those to release hydrogen cyanide gas clouds into the chamber. While instructing the condemned to breathe it in to avoid prolonging the process.
Carbon monoxide is odorless and invisible, the subject (animal or human) would fall asleep, go into a coma, and asphyxiate by blood oxygen depletion in that state. Other than causing an extreme "blush" to the skin (looks purple-ish on dark skin, pink to red on light skin), it leaves a fairly peaceful looking corpse. The condemned could also be helped by a strong sedative, such as Versed ("vehr-sehd"). No wonder so many suicides have chosen carbon dioxide.