It's Satan's asshole.

He's aging backwards because of his contract with Satan.

Bill's able to get the memory to Stan's safe thanks to this, but before he can finish reading it to Gideon,

Bill agrees to go into Stan's mind under the condition that Gideon helps him out later on, to which Gideon agrees.

the identity and the fate of the writer of Journal 3, or uncovering the truth behind Stan's secret

Dipper and Mabel Pines, spending the summer with their grunkle stan in the sleepy town of Gravity

You're an El Buco stan.

I'm an El Buco stan. Does that mean I troll them and want to like hurt them?

And me and this guy Stan, we went to birthday parties and we had no skills. Literally no skills.