As asked. I understand that Hitler had some personal biases against them in warfare, Churchill advised toward their possible use, and Japan used them but not against Allied forces fearing similar retaliation, but why would Stalin refrain from using them against the Germans?
First of, that was kind of mutual agreement: neither side will use chemical weapons in fear that other side will use them. You can find claims that Nazi used chemical warfare in Adjimushkay catacombs, but it's was a by-product of Soviet propaganda.
And also Soviets didn't have enough bombers or other means of sufficient delivery for effective gas attacks (which was one of the factors why Allied 'advise' was seriously considered by Hitler).