IMPORTANT NOTE: This is a temporal state that I use so admins would know how to edit the page. I will restore this back to its original state after it's done.
First introduced in the episode The Hand That Rocks The Mabel, Li'l Gideon first comes to Dipper and Mabel's attention when they both witness a television commercial promoting the "Tent of Telepathy", claiming that the owner of the tent possesses psychic powers. Despite the fact that Grunkle Stan forbids them from going due to Gideon being a rival business competitor, the twins and Soos visit the tent anyway to test the psychic's authenticity. Unsurprisingly, Gideon is a con artist, but clearly too charismatic for most of the audience to tell.However, during the show, Gideon notices Mabel in the audience and visits her the next morning, asking if she wants to join him at his dressing room to perform makeovers, an offer which Mabel accepts. Eventually, however, Gideon expresses his love for Mabel, stating that he wants to be more than just friends; Mabel doesn't quite feel the same for him, but reluctantly agrees to accompany him on a date - on the condition that it's just the one. At a local restaurant, however, Gideon (with the help of a macaw) proposes another date with all other diners spectating, leaving Mabel no choice but to accept. Stan, disapproving of the apparent relationship, decides to talk to Gideon's father in an attempt to call the whole thing off; unfortunately, Bud Gleeful bribes Stan with a large sum of money in exchange for encouraging the relationship between Gideon and Mabel.
Dipper soon realizes that Mabel is starting to crack under the pressure of not being able to refuse Gideon's advances, and offers to break up with him on her behalf. Although Gideon manages to suppress his anger during the meeting, he is convinced that Dipper is attempting to sabotage their relationship and begins to plot his revenge. Bribing Toby Determined with Shandra Jimenez's phone number, he gets Toby to send Dipper on a wild goose chase to Gideon's factory: it's soon revealed that Gideon has a powerful amulet that actually grants him real psychic powers - most prominently telekinesis - and uses this in an attempt on Dipper's life. However, Mabel soon arrives at the factory, and breaks up with Gideon in disgust; for good measure, she not only defeats him with his own amulet, but shatters it soon after so that he can never misuse its power again.
Afterwards, Gideon is seen plotting his revenge with a wooden play-set of the Mystery Shack, complete with miniature versions of Dipper, Mabel, and Grunkle Stan. It is then revealed that he owns another one of the mysterious journals, but this one is marked with the number two, instead of three.
Gideon makes his next appearance in Irrational Treasure. He dresses up as a tomato farmer (as he puts it: "A humble tomato farmer, selling his wares!") on Pioneer Day and takes the opportunity to throw tomatoes at Stan after he's been put in the stocks for the day. He is also the main antagonist of the episode Little Dipper, where he manages to obtain Dipper's size-changing device and plans to use it in order to take revenge on Stan; however, he is ultimately foiled by the shrunken Dipper and Mabel when they tickle him, allowing Stan to casually roll him out of the Mystery Shack.
After spending the last few episodes either harmless or easily foiled, he returns to the role of a main antagonist in the episode Dreamscaperers, in which he summons Bill Cipher in order to learn the combination to Stan's safe and steal the deeds to the Mystery Shack, promising an unspecified favor in return. After infiltrating Stan's mind, Bill is nearly successful in getting the combination, but loses it thanks to the interference of Soos and Mabel, leaving Gideon to call off the deal in a huff. Though Bill automatically attempts revenge, Dipper, Mabel and Soos are able to outmatch him just long enough for him to call a stalemate and leave; exiting the mindscape, the three of them celebrate with Stan - only to discover that Gideon has resorted to blowing open the safe with dynamite and stealing the deed. Now victorious, he uses his ownership of the deed to force the Pines out of the house, and for good measure has Bud Gleeful destroy the Mystery Shack's sign with a wrecking ball, the episode ending on a cliffhanger.
In the season 1 finale episode Gideon Rises, he has completely taken over the Mystery Shack, publicly claiming he plans to remodel it as "Gideon Land." However he really intends to find the first Journal, which he believes is hidden somewhere on the property. After trying and failing to convince the townsfolk that Gideon is a fraud, Dipper tries to call in the Gnomes against him, but Gideon is able to tame them with a powerful whistle and force the Gnomes to carry Dipper and Mabel away. Worse still, Gideon discovers the Journal in Dipper's possession, and gladly takes it for himself, mocking Dipper over his perceived inability to do anything without the book's advice. With Stan now living at Soos's house and unable to support the twins, Dipper and Mabel have no choice but to take a bus back to Piedmont. However, after realizing that the Journal he took from Dipper is the third and not the first as he assumed, Gideon is enraged: believing that the books are a means to open a gateway to ultimate power, he needs all three of them in order to achieve total control over Gravity Falls. Assuming that Dipper has the first Journal - and having commissioned a giant motion capture-controlled robot from Old Man McGucket - he takes control of the Gideon Robot and sets off after the bus in an attempt to capture the Pines Twins.
Though Soos (posing as the bus driver) is able to avoid Gideon for a time, Mabel ends up getting captured by the robot, leaving Dipper behind. Initially believing Gideon's earlier insult, Dipper comes close to giving up altogether, but then charges the robot, smashing through its control room windows and attacking Gideon himself. However, since the robot is motion capture-controlled, the struggle between Gideon and Dipper ultimately sends it plummeting off the edge of a cliff; fortunately, Dipper and Mabel survive thanks to Mabel's grappling hook, and Gideon is able to escape unharmed despite the damage to the robot. With a crowd flocking to the scene to investigate the chaos, he immediately tries to get Sheriff and Deputy Blubs and Durland to arrest the twins, but Stan arrives with evidence confirming that Gideon's psychic abilities are fake: the buttons he'd been giving out were really secret cameras, allowing him to spy on the townsfolk and learn enough details of their private lives to convince them he was really capable of reading minds. With Gideon now unveiled as a fraud, the citizens of Gravity Falls turn on him, and Blubs and Durland place Gideon under arrest - but not before he swears revenge on the Pines Family, promising to be back one day. In the meantime, Stan moves back into the Mystery Shack with the twins in tow - and is revealed to be the owner of Journal #1, and with Dipper and Gideon's Journals in his possession, he now has everything he needs to begin work on the dimensional gateway hidden under the shack.
Gideon next appears in prison during the season 2 premiere, where he notices the unusual phenomena kicked off by Stan's work on the portal.
He reappears in The Stanchurian Candidate, this time secretly orchestrating Bud Gleeful's election bid for the post of Mayor; despite still being imprisoned and under heavy guard, he is able to communicate with his father via a video screen, and clearly hopes to receive a full pardon in the event that Bud wins. However, when Stan begins an election campaign of his own, Bud finds himself outmatched in the debating field, though only because Dipper and Mabel are correcting Stan's approach via mind control; unwilling to see his chance for freedom slip through his fingers, Gideon is forced to make use of a spell he stole from the second Journal (hidden in his hair up until now) and possess Bud's body. He first uses this spell to give Bud an easy win at the final debate of the election, and then to capture both Dipper and Mabel, arranging to blow them up with dynamite; however, Stan is able to rescue them, and in the chaos that follows, Bud loses the election and the Journal page is destroyed. Though apparently defeated, during the end credits, Gideon reveals that he is hiding a summoning circle for Bill Cipher in his cell; completing the circle, he claims he's ready to make a deal - essentially offering his services as Cipher's minion.
When Bill's plan to enter reality succeeds in Weirdmageddon: Part 1, he arranges for Gideon to be broken out of prison to serve as the sheriff of Gravity Falls and the guardian of Mabel's prison bubble - granting him all of eternity to get Mabel to love him. Having befriended most of his fellow prisoners, he also has a gang of his own, complete with monster trucks. When Dipper and Wendy attempt to steal a car from Bud Gleeful's dealership, they are immediately confronted by Gideon's gang; however, Wendy is able to steal the key to the prison bubble from Gideon and lead them on a wild chase through post-apocalyptic Gravity Falls, and though they eventually succeeded in surrounding them, Dipper manages to convince Gideon that he'll never be able to force Mabel to love him. Despite knowing full well that defying Bill will mean facing his wrath, he nonetheless decides to let Dipper free Mabel, and drives off with the rest of the gang to buy him and the others some time in a battle with Bill.
In Weirdmageddon 2: Escape from Reality, Bill learns of Gideon's betrayal but is largely unmoved by it, believing that the bubble will simply trap anyone attempting to rescue Mabel from it. Nonetheless, he orders the Henchmaniacs to fetch Gideon for punishment - though Gideon himself is not seen at any point in the episode.
In the series finale Weirdmageddon 3: Take Back the Falls, Gideon is locked in a birdcage while his gang members are petrified and added to Bill's throne of human agony; for good measure, he then is forced to do a cute dance against his will for Bill's pleasure - until Dipper and Mabel arrive in the Shacktron to free him and the other petrified victims. Ultimately, Gideon stays behind in the Fearamid to help Dipper, Mabel and their friends to stop Bill, proving instrumental in forming the Zodiac Wheel that Ford Pines believes will be able to stop Bill once and for all; unfortunately, an argument between Stan and Ford results in Bill ambushing the group, destroying the wheel and transforming everyone except the Pines family into tapestries for his palace. Fortunately, Stan and Ford are able to defeat Bill by tricking him into entering Stan's mind while Ford uses the Memory Eraser to wipe his brain, effectively destroying Bill with all his memories.
Following Bill's death, Gideon and the others are turned back to normal and the damage done by Weirdmageddon is reversed, much to the relief of all parties concerned. Gideon and his family later attend Dipper and Mabel's 13th birthday party, and Gideon admits that he's changed for the better. However, he still maintains a notably vindictive streak: when he's shown trying to ride on a skateboard, one kid makes fun of him, and an offended Gideon sends his gang members to beat up the kid in retaliation.