Sorry to rain about your parrade about poison gas, but the first one who advocated using chlorine gas (you know, the same that would later be actually used in WW1) against the enemy was... John Doughty, a school teacher from New York, during the American Civil War. Later at the Hague Conference in 1899, guess who voted against forbidding shells filled with asphixiating gas? Right, the USA, on grounds that, "the inventiveness of Americans should not be restricted in the development of new weapons."
So, sad to say,
1. Germany didn't invent that idea. That merit goes to the "inventiveness of Americans".
2. The idea was actually applied some decade and a half after the whole world was already seriously aware of the possibility.
(And just so I don't bash the USA alone, other countries had their own apologists of poison gas attacks. I seem to remember some French utopianism (or dystopianism) where future wars would be won by massive poison gas attacks and cavalry charges on bycicles.)
Or do you mean Gas Chambers? You know who invented that one? The USA again. Execution by gas chamber was first used in the 1920's in the USA.
So again, Germany sadly can't claim to have invented that one, and merely copied the fruits of the good ol' "inventiveness of Americans." When it comes to killing each other, you can trust America to have the good ideas first. Kudos, and all that.