Gravity Falls is an animated TV show created by Alex Hirsch (a good friend of Rick and Morty co-creator Justin Roiland) that aired on Disney Channel, then moved to Disney XD. The show is about twins Dipper and Mabel Pines, who spend their summer at their Grunkle Stan's Mystery Shack, in the town of Gravity Falls, Oregon, where they solve mysteries and face many paranormal phenomenons.
Many Easter eggs referencing Gravity Falls may be found in Rick and Morty. Reciprocally, Gravity Falls (known for its hidden messages, codes and Easter eggs) contains several references to Rick and Morty. It should be noted that inter-dimensional travel exists in both shows, creating the possibility that both are set in the same multiverse.
Gravity Falls in Rick and Morty
In the episode "Close Rick-counters of the Rick Kind", Rick opens up a bunch of portals to other dimensions. One of the portals has a mug with a question mark, a pen and a notepad come out of it. In the Gravity Falls episode "Society of the Blind Eye", the episode's ending credits gag reveals that Stan is the one losing those three exact things, by having them get sucked into a Universe Portal.
In the episode "Big Trouble in Little Sanchez", a picture of Gravity Falls antagonist Bill Cipher can be seen on a monitor while Beth and Jerry go to counseling.
In the episode "The Rickshank Rickdemption", where Summer and Morty are brought to the Citadel of Ricks. Morty explains to Summer that this is where all the Ricks and Mortys from multiple alternate worlds hide from the Galactic Federation. While Morty tells her this, in the background for a few small frames are a pair of Morty twins: a boy and a girl. The boy is wearing Dipper Pines's pine tree hat while the girl is wearing Mabel Pines's headband and hairstyle. In another frame of that scene, a Morty is seen holding a copy of a book similar to Journal 3.
In the post-credits scene of "The Whirly Dirly Conspiracy", a low contrast graffiti resembling Bill Cipher may be seen at the top left part of the bridge's side.
In the episode "Rest and Ricklaxation", a decaying version of 8 Ball (one of Bill's friends from the Nightmare Realm) can be seen in the toxin tank.
In the episode "Morty's Mind Blowers", there's a room with archives of all the experiences Morty have begged Rick to remove from Morty's life. There's one blue bulb with the word "STANFORD". This is a reference to Ford Pines, one of the main characters of Gravity Falls, known as "The Author" of Journal 3. In the same shot (18:22) there is also a bulb labeled "Bill C.", which is a reference to Bill Cipher. Also in the opening dimension, there are statuettes of what appears to be the Truth Tortoise holding up a triangle with the All Seeing Eye. This could be a reference to Gravity Falls character Bill Cipher, with the triangle having the same eyelashes as him. Alex Hirsch later responded it was simply the Eye of Providence and tweeted how to differentiate the two. In addition, the back side of the page handed out by Zick Zack contains a pattern resembling the blueprints of Gravity Fall's Interdimensional Portal.
In the interactive game in Instagram "Rick and Morty Rickstaverse", by going into the federal prison, in a room there's a monitor with an image of Bill Cipher and the symbol of Journal 3. One picture below it, you can see a collection of Rick's items in a tray, next to one labeled "???". It contains the three items (a mug, a pen and a notepad) that Grunkle Stan loses into the portal during the episode "Society of the Blind Eye" credits.
Rick and Morty in Gravity Falls
In the real-life published version of Journal 3, which is canonically identical to the journal featured in the Gravity Falls series, there's a silhouetted Plumbus and, on another page, a Zigerionon (What Does It Mean?!). It also has a page with a copy of Ford's WANTED poster, with an encoded message (in one of Gravity Falls ciphers) on the corner reading "RICK WAS HERE". On the secondary page (My Travels), Ford mentions fist fighting with a talking chair, a reference to the Furniture Universe.
In the published and canonical book Gravity Falls: Lost Legends, the first story, Face It, contains a memory capsule labelled "Rick S." on it, a reference to Rick Sanchez. Later in the same story, Mr. What’s His Face is shown to be in the possession of the severed faces of "Evil" Rick and an unnamed Morty, whilst in the third story, Don't Dimension It, Mortabel is visible in the background of Dimension MAB-3L as Mabel Pines is encountering other realities' versions of herself.