gravy already has lots of salt in it.

By all means use gravy, not too much but dont put salt in it.

By the sounds of it hes liking the smell of food more than the taste, as cat food smells more, try putting more cat food in to a dog food then less each day or so, so you end up with just dog food over time, with a little gravy as well.

by the way human chocolate isnt good for dogs at all, do a google if you need proof on that.