The symbols found in Journal 3, as well as other Gravity Falls publications, presumably written by Bill Cipher.
Gravity Falls and everything related to it belong to Alex Hirsch / Disney.
The symbols found in Gravity Falls: Journal 3, created by Bill Cipher.
Gravity Falls and everything to do with it are the property of Disney.
-- Spoilers --
Bill drew this language in secret messages throughout the Journal, as well as a wheel which can be used for decoding.
The symbols found in Gravity Falls: Journal 3, created by the Author. At one point, the symbols can be translated to read "I call this Fordese."