On the journal page facing the page for Island Head Beast, the author writes
"There is no other place I would rather be in Gravity Falls than the lake. It reminds me of my childhood ...Glass Shard Beach.
The flora seen throughout the lake area is tropical. After some of my adventures in this region, I am surprised not to discover something more unusual.
common lake plants" (zoomable image)
Obviously Hirsh has inserted a lot of references to his own life and real world places and things that don't exactly match up the real world counterparts - Pitt soda instead of Mr. Pibb, after one of the directors of the show, for example. But, after a quick google search, there is a real life Glass Beach covered in beach glass shards, near FT. Bragg CA, which happens to be a 3 hour drive from Piedmont (the twins' hometown).
If the true author comes not only from California, but relatively near where the twins are from (close enough to be a vacation destination or a family's location drift over a few generations), what purpose would that serve the plot?
One idea is that Grunkle Stan has a brother, by dint of being a great uncle with the same last name as the twins. It's possible that Dipper and Mabel's grandfather was the older of the two, grew up in California instead of Jersey, and wrote the journals. In this circumstance, Grunkle Stan's motive in building the portal would be either to continue his brother's work or rescue him from being stuck on the other side of the portal. Lying to the twins about his knowledge of the paranormal makes a lot of sense if he showed up and joined the Order of the Holy Mackerel just before his brother dissappeared into another dimension. In this scenario, his changing hat symbol is just him switching back and forth between his brother's secret society hat and his own.
- Holes: Why the cross country move? How would one brother grow up on one US coast and one on the other? From the timing of the journals it might make more sense for Grunkle Stan's father, the twins' great-grandfather, or an earlier relative to have been the author. But not too early, due to the July 1982 calendar in the room with the electron carpet.
Another (heavy heavy meta) possibility is that the "portal" to another dimension is going to reveal to the characters that they're actually cartoon characters, and the author writing the books is the actual author of the story, Alex Hirsch. Supporting evidence can be found in dreamscaperes where Bill pushes all the imagined weapons away, and all thats left is a pure white background - historically used by cartoonists to break the fourth wall, Bill's assertion not a minute later in the same episode that reality is an illusion and the universe is a hologram, the fact that Alex's last name initial (H) and birthday date (618) show up more often than Bill does in the background symbolism, the demonstrated power of imagination in the mindscape (both the protagonists creatively solving problems and literally imagining solutions to their problems out of nowhere) and lastly the fact that characters have been known to say and do things without understanding why they did - such as when shrunken Soos wonders out loud why he revealed Stan's location to Gideon in "Little Dipper." heh.