Is your objection to execution by gas, or execution at all?
I agree that the death penalty is wrong. But if you are going to have the death penalty gas is probably the most humane way to do it.
The thing is its the association with killing people that is the important part here... making the gas the important part and not the killing is really just a bad take.
Killing people like the nazis is bad... making people breath a gas (without killing them) is not bad, despite the fact that the nazis made people breath gases. However making people breath gases that kill them is wrong, but because it kills them, not because they are breathing gases.