This page is all about the journals.
Involvement
Old Man McGucket
A picture (which is all but proven to be fake) was leaked near the end of season 1 which showed a younger, 6-fingered Old Man McGucket writing Journal #3. According to my theory, he did in fact help Stanley write the journals.
Gideon
Gideon owned Journal #2 throughout season 1. Although the method by which he lost it is clear (Stan subliminally took it from his pocket), it is still unknown how he acquired the journal or how much he really knows about it. My guess is that he stumbled upon it and knows little more about it than Dipper.
Stanford
Stanford has obviously been searching for the journals for many years. He owned Journal #1 for at least a portion of time, but only acquired the other two at the end of Gideon Rises. My theory is that he helped write the journals, along with Stanley and McGucket. At some point they each buried a journal, Stanford having #1, Stanley #2, and McGucket #3, but later events caused Stanford to dig up his journal.
Stanley
Stanley was probably involved with the journals in some way. My guess is that he was the main author, along with Stanford and McGucket, and was eventually kidnapped by Bill and imprisoned in an alternate dimension, hence prompting his brother to attempt to open the portal and save him.
Dipper
Dipper does not seem to have much involvement with the journals (except his possession of #3). He stumbled upon it and does not (yet) know the secrets of the journals.
The Royal Order of the Holy Mackerel
I believe that the authors of the journals, the Stans and McGucket, were all members of the ROotHM. The other members could be involved somehow (maybe secretly helping Stanford with his quest to save his brother?)
Soos
With Soos' concealed smarts, I wouldn't be surprised if he knows a thing or two about the journals. He obviously knows about Dipper's journal, and wants to know who wrote them. He may show his knowledge later in the series...who knows?
Bill
Bill is on a page in both Journals 2 and 3, at least. From his pages, he apparently can appear to be an honest and trustworthy individual, but will later prove that assumption wrong. I think the Stan twins and McGucket encountered Bill in 1982, and he helped them with something, but later turned on them.
the portal
I think Stanley discovered one day between 1954 and 1972 that they could build a portal. He realized that if someone opened the portal, it would unleash a gateway to infinite new worlds, so he split up calculations for it between the three journals. He and Stanford traveled around the world, gathering parts for the portal. The lab beneath the Mystery Shack was built to house the portal.
similarities
In Tourist Trapped, one of the first pages of Dipper's journal has the alchemical symbols for composition, pulverize, digestion, and fusion. This, as we find out in Gideon Rises, is the passcode to Stan's lab.
