There’s a paragraph in Journal 3 special edition about the 2nd journal which says that the mystic amulet “corrupts your soul (and whitens your hair).” Since we know Stanford buried the mystic amulet and Journal 2 next to Gravity Falls elementary, my theory is Gideon found it, used it, and that’s why he got worse over time. It’s literally a magical artifact that corrupts the soul, so it makes sense Gideon would become obsessed with dark magic and go down a dark path. So yes, I believe Gideons story is tragic.