So you had ammonia readings in the tank yesterday? Gouramis are not particulary hardy fish and its entirely likely he has ammonia poisoning. I would do frequent water changes to maintain the 0 ammonia reading you currently have.
i would just keep the water as pristine as possible. Closely monitor his eating as well, you may want to mix his food with some crushed garlic if he isn't eating well to stimulate his appetite.
I agree, keep doing small daily water changes. That's the best way to heal the gourami as well was the cory's lost barbels. It sounds like the gourami is doing better since the water change so I bet the high ammonia is to blame.