Drugs have a specific half life, meaning a set amount of time it remains in the animals system before it is no longer there or effective. On top of that the animals (speculating here) would not need to tested individually, they could test batches of processed meat.
As for gassing being inhumane, it is humane enough for feeder rodents and it is far more humane than being eaten alive by a predator (nature's way). Trying to say there is a difference between gassing a rat and gassing a cat is nothing more than anthropomorphizing.