At the end of book #1, it states that the journal continues on to journal #2 yet when Lil'Gideon had journal #2 he didn't know there was an existence of journal #3.
Gravity Falls: : Journal 3
Journal 1 | Gravity Falls Wiki | Fandom
Gravity Falls Journal : 7 Steps - Instructables
Gravity Falls Journal 1-2 Unofficial, 20x27 Cm, English, Invisible Ink, Gravity Falls Book - Etsy UK