I prefer that states have the option of the firing squad: cheap, reliable, effective.
Gases however, have a problem. They work off the assumption that the condemned will hold their breath, then gulp in the gas. However, in Arizona, a prisoner just continued to breathe normally, cyanide gas, and died a horrific death of suffocation, quite obvious to observers.
Cyanogen induced respiratory paralysis-central nervous system-can’t draw a breath.