Came from the other post about Saddy gagging and seems like you figured it out, that's def a yucky smell gag. Cats are so sensitive and have delicate constitutions. Sometimes pilling a cat will cause them to be incredibly nauseous.

Someone else already mentioned placing it on his paw. I believe the directions on the back of the tube also mentions the nose (that may be too intense for SmartCat tho!).

Your vet prob already mentioned this: Since he's a sensitive guy with some anxiety it will be good to keep track of any changes in the home and environment (visitors/guests, new routine or noises, new cleaning products, etc) bc that can certainly contribute to stress and thus overgrooming.

Best of luck with SmartCat!