Yes, gizzards are offal (they're a form of second stomach that birds and a few other creatures have).
However, there is no single internal organ from any species of animal that can cover off all your offal needs. They're all rich in different vitamins and minerals, with differing properties. Offal needs to be a range of organs, not just one thing. Not every meal, but over time, you would need to include other organs in the diet.