A Greatest Hits copy entails that the game be sold for a cheaper price. There's no special content in the package. Mainly, it's just to show that the game sold plenty of copies, enough to make it cheaper for future consumers who are buying the product.
Differently, the Game of the Year Edition for Dead Island includes bonus content. You'll be getting all available DLC for the game, including the Ryder White campaign that came out early last year.