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What examples are there of the Empire using poison gas?

I can only think of one, from one of the X-Wing books, where the Empire gases a hallway between a computer terminal and an entrance.

An Imperial Colonel uses a poison gas against the natives of a planet (Icarii ?) in the comic Boba Fett: Enemy of the Empire.

It doesn't count, but poison gas was used in The Phantom Menace.

Are you looking for chemical weapons specifically or are you trying to find examples of biological weapons as well? The Empire appeared to be more interested in using manufactured viruses and diseases as opposed to chemical weapons such as the Trihexalon gas used by the CIS.