I agree with the first comment.
This isn't my theroy but this is what I heard, in the theme song thing when u pause it at the right time ppl say its bigfoot, I saw a video that said it might be Grunkle Stan.
I believe that it has to do with "The Royal Order of Holy Mackerel". There is a tapestry behind Stan's poker table in the game "Mystery Shack Mystery" and when you scroll over the it, it says "The Royal Order of Holy Mackerel". On the tapestry is a picture of a symbol similar to the one on Grunkle Stan's fez.
One of my theories is that Stan is in this secret order and is protecting the secret of Gravity Falls... whatever that is. Maybe the group meets in the basement behind the vending machine.
I think that "Stan Is Not What He Seems" thingy means that when Grunkle Stan was 12, he was just like Dipper, he owns the first journal and wrote all 3 (Well, maybe there are more, but this isn't one of my theories.). Oh, and remember in Dipper's journal? Trust no one? Don't trust Grunkle Stan about who he says he is!