The number 618 is frequently referenced throughout the show as a nod to show writer Alex Hirsch's birthday, June 18th.
The number is seen in virtually every episode. The recurring examples include the address of the mystery shack being 618 Gopher Road. The cash register within the mystery shack usually displays the figure £6.18. In the attic, an old game has the score of 8190 which reads 0618 upside down. Similarly, the Whack A Customer game has a number 819 which, when flipped, reads 618. Gideon's school ID number is 618.
There are obviously several more but there's just too many to list. Next time you watch an episode of Gravity Falls, look out for the number 618.