The GT game will be fully compatible with the current DBZ game, running on the same basic game engine. There will be a different card back for GT, however, so card sleeves will become important in tournaments if you want to mix Z and GT cards.
Even though GT will be an extension to DBZ, this new game will be able to stand on it's own as a separate game. So, new Players that just want to stick to GT can do so, while existing Players that are hungry for even more cards will get them
The first GT set will be both a starter and booster expansion and will be heading your way after Kid Buu Saga in early 2004. GT will definitely make the game as you know it bigger, beefier and better than ever!