Notes:

My edit using this template.
Source Gravity Falls episodes for each panel:
Circumventing a time curse: "The Time Traveler's Pig"
Solving a What-The-Heck-Ahedron: "Little Gift Shop of Horrors"
Finishing the Grand Unified Theory of Weirdness: "The Last Mabelcorn"
Finding out who stole the capers: "Carpet Diem"