Experience Tumblr like never before
This is… genius oh my god.
I made a past post showing parallels between Gideon & Bill a while back but the main parallel I noticed are their relationships to Ford and Mabel:
They used to be close to Ford/Mabel before their messed up actions caused a falling out between them
Gave them chances to join their side
Held them captive during the course of Weirdmaggedon
Now this could've been accidental and me just reading into things but the Book of Bill had this recurring idea of Bill being Ford's toxic, abusive ex. It just really cemented this parallel and the idea of Bill being Ford's Gideon.
IN ADDITION - While this might be reaching, I think with this idea in mind, there's an additional way to read to Gideon's line:
"Bill explained it to me nice and simple. She was always destined to be mine. Now that I have her trapped in a cage, she'll learn to love me. I have an eternity to wait"
Another way to read that is, Bill's projecting onto Gideon & Mabel a bit and this was what he initially had planned with Ford: Keep them trapped, and use the endless amounts of time they have to make him "learn" to love him, but then the barrier surrounding Gravity Falls derailed that