Gabe calls himself "a master of puppets", which is reference to the Mettalica song Master of Puppets. In addition his poster says "he'll pull your heart strings" a reference to one line from the song, "master of puppets, I'm pulling your strings".
Soos' poster "HE'S GOT ATTI-CUBE" on the door of his room has a COOLCUBE logo at the top that is a cube made of C's, a parody of the Nintendo GameCube whose cube is made of G's. The running cube on the poster also resembles a Rubik's Cube and 7-Up's old mascot, Cool Spot.
The fact that Giffany killed her creators after becoming sentient is a nod to GLaDOS from the Portal series. Giffany's animation depicting a bunch of stick figures at computers greatly resembles similar animations played in Portal 2.
On Giffany's screen, the binary code means "SPACEJAMTWO." This is a reference to the film Space Jam and how it may be getting a sequel sometime soon.
Giffany's pesonality archetype is known as "Yandere".
When Soos mentioned that Giffany's father is the "octopus man", this is the reference to the tentacle Hentai.
Soos says he doesn't know whether to pronounce Giffany's name as "Giffany" with a hard G, or "Jiffany." This is a reference to the common internet dispute about whether "GIF," a file format, is pronounced with a hard or soft G.
The ending credits sequence is a reference to the film The Hangover.
The cryptogram at the end slide is a parody of the theme song of the DiC/CWi dub of the anime Sailor Moon. "WINNING HEARTS BY DAYLIGHT! / POSSESSING ROBOTS BY MOONLIGHT! / HER EMOTIONAL BAGGAGE IS A REAL FRIGHT! / SHE HAS THE ONE NAME GIFFANY." Similarly, the first four lines of the DiC/CWi's Sailor Moon theme song are "Fighting evil by moonlight / Winning love by daylight / Never running from a real fight! (or 'With her Sailor Scouts to help fight' in a later verse) / She is the one named Sailor Moon!"
The show "Ghost Harassers" is a reference to the movie Ghostbusters. Dipper also mentions "bust[ing] ghosts" several times in the episode.
Used to be About History Channel is a reference to the History Channel. The logo is also similar to the History Channel logo. The name is also a reference to how the channel very rarely runs any actual historical documentaries anymore.
The basketball player is a spoof of the Oregon Ducks, a college team for many sports.
In the scene where Dipper and Pacifica are hiding from the Lumberjack ghost, a painting is seen on the wall depicting a duck hunt. The way in which it is presented is an allusion to Nintendo'sDuck Hunt from 1984.
One of the verses in the exorcism is "ain't afraidus no ghostus," which is reference to Ghostbusters theme, in which one of the lines is "I ain't afraid of no ghost."
Watching a portrait of a man and then re-watching it only to find that he's gone from the frame parallels a scene from the Scooby-Doo episode Haunted House Hang-Up, where the head of the man in the portrait is gone.
The "Mothgar" poster in Stanley and Stanford's childhood room is based on the American theatrical poster for the 1961 film Mothra.
There is a poster in McGucket's old home, which shows a colorful, bitten-off strawberry similar to the Apple logo, and the words "Ponder alternatively," which is a play on Apple's slogan "Think differently."
Furthuring the Apple Computers / Fiddleford H. McGucket connection is the fact that his and Steve Jobs' houses are virtually identical.
College Ford was shown in learning progression montage. During that montage, two posters are shown on his dorm room wall. They are the iconic Nicola Tesla and Carl Sagan, two great thinkers and doers of recent history.
The magazine on Stanley's bed titled "Wuh?" is a reference to Mad Magazine. The person on the cover of the magazine is a reference to Alfred E. Neuman.
The time his car was grabbed by a giant arm is likely an allusion to the '90s movie the “iron giant”