book #2 / rivalry with Stan
Gideon and Stan seem to know each other about as well as any two foes would. For example, in The Deep End, Gideon predicts Stan's plan to sit on the chair all night and coats the entire chair with glue.
Season 2
Gideon will definitely play a role in Season 2. Even though he's in prison, he's still a threat to the Pines family.
In Little Dipper, Gideon tries to think up a way to exact revenge on the Pines family "but not enough to harm 'em." And he threatens Stan with the cursed Egyptian super termites, but he was obviously bluffing. Why? Is it because he needs the Shack? Maybe because he doesn't want to hurt Mabel? Or is it because he knows that he will eventually need them?
In Gravity Falls TV Episode 1, Gideon is on a reality TV show called Li'l Gideon's Big House. This shows just how dangerous he still is. The prisoners love his psychic act, are more forgiving than the townsfolk, and have been looking for a new leader, according to Alex Hirsch's AMA.
As of the first hiatus of season 2, (after The Love God), Gideon has only made one official appearance. It was in Scaryoke for a fraction of a second. However, Disney XD has shown a series of commercials called Creepy Love Letters From Lil' Gideon. They don't mean much. Also, there is one letter from him in Dipper and Mabel's Guide to Mystery and Nonstop Fun! It doesn't mean much at first glance, but put the first letter of each line in order and it says "HELP ME ESCAPE OR BE DESTROYED."
There are rumors that Gideon will return soon. We can be 100% confident that Gideon will return this season.
Ma-Gid-bel-eon
Gideon has strong romantic feelings for Mabel, as shown most plainly in the episode The Hand That Rocks the Mabel. This could be an easy weakness for the Pines family to exploit, but he has shown willingness to throw the baby out with the bathwater, so to speak.
"I got a good feelin' about that kid"
Old Man McGucket is seen twice working with Gideon. My personal theory is that he helped write the journals, knows that Gideon has Journal #2, and is secretly trying to figure out who has the other two (because when Bill messed up his mind, he forgot to erase the memories of him writing the journals.)
relations with his father
Gideon seems to have complete control over his father. He apparently could "buy and sell" Bud. But how? Could he be referring to the show, in which one slip could send the family spiraling downwards? Or maybe the journal, with which he could keep his father in check?
Bud's reaction is interesting too: he just submits without a second thought, as shown in The Hand That Rocks the Mabel, Little Dipper, and Gideon Rises. Although, upon Gideon's arrest, Bud was quoted as saying that "he could use a time-out." We finally learn why this is in Society of the Blind Eye: Bud keeps forgetting about Gideon's tantrums.
Blendin's Game gives us a glimpse back in time. It shows that Gideon was a newborn baby 10 years ago. It also reminds me of a YouTube video, which describes Gideon as of The Hand That Rocks the Mabel as a 9-year old. So Gideon obviously has a summer birthday. Not only that, it's not unlikely that he spent his 10th birthday in prison!
other
Gideon is probably an albino. That's a rare skin condition, causing the victim to have unusually white and pale skin, as well as to burn rather easily. This is the reason we see Gideon applying lots of sunscreen in The Deep End.
The contents of Gideon's pockets upon his arrest were as follows:
illegible list
wallet
keys
2 crumpled papers
hard candy
ax (like the one seen in Headhunters)
red screwdriver (like the one Stan was missing in The Time Traveler's Pig)
picture of Mabel
Journal #2
property deed - 618 Gopher Road (lands on top of journal)
