Good Morning and welcome to Just answer. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
There is no evidence that Benadryl will cause any harm to the liver even given long term. This is metabolized in the liver and if there is any liver damage, this medication is not recommended because it will not properly metabolize this medication.
Dogs with Cushings Disease will generally drink excessively and have a voracious appetite. From your description, I doubt that Gus is suffering from this disease. Liver damage may cause an increase in liver enzymes caused by age.