We didn't really need help to figure out that Play on Wii Mario Tennis GC (or Mario Power Tennis, as Camelot's mascot tennis game is known over here) would use Wiimote motion controls, since that is, like one hundred percent of the point of the Play on Wii series (and Wii tennis games, for that matter). But we welcome solid information anyway, which Japanese online retailer Neowing has provided.
According to the product description, Mario Tennis GC enables players to wave the Wii Remote to execute forehand and backhand shots and direct the ball toward the left or right side of the court. Motion will also determine the addition of topspin and slice spin. Buttons are still used to deploy each character's unique special moves.
If the (possible) second coming of Wii Sports tennis doesn't excite you, hang on to your GameCube version: Play on Wii Mario Tennis GC does not support GameCube controllers.