618

 is a recurring number in Gravity Falls. It is significant because it is based on the date, June 18th, the birthday of series creator, Alex Hirsch, and his twin sister, Ariel Hirsch.

Instances
The following are instances 618 is used in the show:
 * The show's main production code is G618.
 * E.g., Dipper and Mabel's rough model sheets are labeled "618G99.01."
 * E.g., An episode production code starts with 618, such as "618G-107" for "Tourist Trapped."
 * The Mystery Shack's address is 618 Gopher Road.
 * The cash register in the Mystery Shack gift shop is always seen as reading $6.18.
 * In Tumbleweed Terror, the score on the barrel is 618.
 * Gideon Gleeful's school ID number is 618.
 * The Whack a Customer game in Dipper and Mabel's room has the number 819 on it, which, when turned upside down, is 618.
 * One of the blocks a baby chews on in the television show, Baby Fights, has the numbers 6, 1, and 8 on it.
 * The introduction page of book Journal 3 has the date June 18th and the source of 618.
 * When Stan was in a Colombian prison, his prison ID number was S618.
 * The number 816, the number 618 backwards, appears multiple times on Bill Cipher's page in Journal 2.
 * When Bill becomes angry, his eye flashes six symbols. The third, fourth, and fifth in the sequence are Thai numerals that spell out 6 1 8.
 * The number 618 appears on a post-it note with the reminder "TK 618 Return 7/3 GF Library."
 * In a page of 3, the numbers 6 1 5+3 can be seen.
 * The greek letter phi, a representation for the number 1.618, is seen on a paper on Stan's desk in "Gideon Rises."