Ubisoft is making a Gravity Falls game for 3DS using the Rayman engine
The show's creator is on board too

Disney has teamed up with Ubisoft to bring Gravity Falls: Legend of the Gnome Gemulets to 3DS.
Yes, it's based on the Disney channel's popular animated show and its creator Alex Hirsch is involved in the project.
Just as good news is that it'll be a 2D platformer made in the UbiArt engine meaning it should play well and look pretty good. That's the same engine that enabled the recent Rayman Legends, Child of Light, and Valiant Hearts.
You'll play as both Dipper and Mabel Pines, switching between the two on the fly, making use of their different abilities as they attempt to find gnome artifacts to restore Gnome Kingdom.
Dipper carries Gnome Battle Cuffs that enable him to dish out close-range attacks. He can also search for hidden items using his torch.

Mabel, on the other hand, has a grappling hook to get herself to higher platforms, as well as issue long-range attacks.
It'll have some Metroidvania-style exploration to it as you revisit areas with new items either found or purchased in the store.
The game should be available in the 3DS eShop this autumn.