Assorted headcanons for Gideon Gleeful since he doesn't get nearly enough love in the fandom!
Contains some AU junk as well for the Mystery Seekers AU/Regiscout! Hi MYSTS
Genuinely learning to be a better person since Weirdmageddon. It takes time, but he's dedicated and motivated.
Had trivial little mistakes and various other things wiped from his mind by his father when he was much younger. He holds no malice towards Bud for this because he never figured it out. The mind may not directly remember, but events like that are imprinted deeply within a person's psyche. The former also contributed to the rapid whitening of his hair.
On the topic of his white hair, it had initially started as Canities Subita - caused by the stress he inadvertently put his body through* when he first found the Journal and stayed up for several nights analyzing its secrets. It worsened with a combination of the memory gun and the usage of the amulet.
* Like Dipper, he has a sort of innocent dissonance when deeply entranced in activity. He doesn't notice any of his body's needs when focused, which is a contributing factor to the Marie Antoinette Syndrome.
While he was never directly psychic, he suffers as an empath. (I say "suffers" because I am one too and it is HELL sometimes.) Surrounded by the glum of Gravity Falls and the often almost depressive attitude of its inhabitants can definitely put a strain on any empaths. Receiving the amulet only amplifies the intensity of empathic traits, which could definitely turn getting shoulder-checked by a bitter stranger into a much more miserable experience, especially if you're on the receiving end of their poorly hidden annoyance at you.
Adding to above, his father notices his misery and has the idea to - instead of talking to him or finding someone he can talk to in order to learn coping mechanisms - throw him into the spotlight and give the people of Gravity Falls something or someone to be happy for.
He never actually had a crush on Mabel. It was moreso a gender envy and general admiration of how unabashedly proud she was to express herself. This is why he refers to her as a "kindred spirit" when he approaches her at the Shack for the first time. He only thinks it's love because he's surrounded by people shipping the two, and because he also doesn't have a stable or healthy example of romance in his life, like Mabel.
He used to be very close with his mother, but after Bud starts increasing the usage of the memory gun on both his spouse and son, and Gideon turning to the Journal as a means of escapism, they start to drift away.
Surprisingly it's talking with Pacifica that helps him figure out herself. They both learn to confide in eachother; Pacifica is more in tune with reminding the both of them to take physical care of themselves, while Gideon is more focused on the emotional needs as an empath.
He hates performing for others with a passion, due to being forced to since his earlier youth.
Started learning French to distract himself from needing to perform.