When does Gravity Falls return?

The series follows the adventures of Dipper Pines (Jason Ritter) and his twin sister Mabel (Kristen Schaal) in a mysterious town full of paranormal incidents and supernatural creatures. The series premiered on June 15, 2012, and ran until February 15, 2016, with a one-hour finale, "Weirdmageddon 3: Take Back the Falls" Gravity Falls received critical acclaim for its writing, characters, voice acting, animation, and humor.
Dipper and Mabel Pines are dropped off from their home in Piedmont, California, to the fictitious town of Gravity Falls, Roadkill County, Oregon, to spend the summer with their great uncle Stan Pines. Things are not what they seem in this small town, and with the help of a mysterious journal that Dipper finds in the forest, they begin unraveling the town's mysteries.