Gastrodon is a sea slug-like Pokémon, and appears much the same as its pre-evolution, but it gains four more stubby legs and is noticeably larger. They both have plain black eyes with white irises, with a third eye in the center of their forehead. Like Shellos, Gastrodon has two forms.
EDIT: I looked into it a bit more, and whenever Gastrodon does a special move in the anime, like Bubble Beam, that third eye glows along with the other two, so I'd say it's safe to say it's indeed a third eye)
Hope I could help!