When does Gravity Falls return?

The series premiered on June 15, 2012, and ran until February 15, 2016. It follows the adventures of Dipper Pines (Jason Ritter) and his twin sister Mabel (Kristen Schaal) in Gravity Falls, Oregon. The series won two Emmy Awards, three Annie Awards, and a BAFTA Children's Award, among various other wins and nominations.
Twins 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 Dipper finds in the forest, they begin unraveling the town's mysteries.