From http://www.gamespot....i/1100-6428774/
As Ubisoft explains in a press release, "The game delivers Gravity Falls' trademark quirky humor in an authentic and interactive way, letting fans play as Dipper and Mabel as they set out to solve mysteries and restore order to Gnome Kingdom. Along their journey, players will encounter many more characters from the show and will explore locations including the town of Gravity Falls, the Mystery Shack, and more. Hirsch also designed the game's original key art."
Notably, the game will run on the UbiArt engine, which powers Rayman: Legends, Child of Light, and Valiant Hearts: The Great War. In the vein of those other games, Gravity Falls will be a 2D sidescrolling platformer. From the reveal trailer, it looks like the game will have Metroidvania elements, as well. There's a store to purchase items at, and the video implies that you'll be able to return to locations you've already visited.
The game launches this fall for Nintendo 3DS