A free app for Android, by Emr Games.

Dipper and Mabel Pines are sent to spend the summer with their great-uncle, Grunkle Stan, in Gravity Falls, Oregon. Grunkle Stan employs the kids to help him run The Mystery Shack, his tourist trap. The twins try to adjust to their strange surroundings, but they sense something strange about Gravity Falls and begin to uncover its secrets. When Dipper discovers a cryptic journal containing clues to the town’s mysteries, he and Mabel use it, along with their fervent desire to defeat evil, to combat the impending mysteries that surround them.

And in this Gravity Falls-themed game, you will figure out what’s going on and try to stop the bill ciphers that are coming from the portal while you keep an eye out for more book pages.