This chapter is by GirlOfManyFandoms, who is writing this on International Harry Potter Day.
'Ello! I am fangirling over Harry Potter, so I decided to bring Gravity Falls in on the fangirl session. Woo Hoo!
So I know the main theory on the author of the Journals is that it was Old Man McGucket. New theory time! It ain't Old Man McG. It is...
So after Harry defeated Voldy, he traveled to the American Ministry of Magic, which happens to be in Gravity Falls, Oregon, because that is convenient to my theory. And while there, he began to investigate all the mysteries of the town, which are all actually wizards messing around.
Oh... You thought he was dead.
He no longer goes by the name Voldemort. He is now Bill Cipher. (That may be a bit of a turn-off, Prez...) He found Harry, and he kidnapped him, trapping him in the Dreamscape, because I refuse to let Harry be killed, even in a crack theory.
Lastly, the red on the Journal pages...
That is not blood.
Anyone who has seen A Very Potter Sequel should know that Red Vines can do anything. I mean, what can't they do? They transformed themselves into a juice, splattering the pages.
Ron Weasley and Hermione Granger died twelve years beforehand due to a Wizardy illness, and they we reincarnated as Dipper and Mabel Pines. (I just shuddered. That's pretty much Pinecst. Ugh...)
Anyway, Ron/Dipper still loves Red Vines, so when he found Journal Three, it was because the Red Vines could sense there was a Red Vine Lover in their presence, and they allowed him to find the Journal. He and Hermione/Mabel need to discover how to get back to Harry. That is what Gravity Falls is really about.
This theory shouldn't be taken seriously, so don't freak. This is just me, being a happy fangirl.