Pacifica's Alt Clothes from Northwest Mansion Noir by Pacifica-Fan-Club, journal
Pacifica's Alt Clothes from Northwest Mansion Noir
I found this in Gravity Falls Wiki. Turns out Matt Braly, aka a director and storyboard artist for Gravity Falls, said in his twitter It's "Northwest Mansion concept art. Explorations by the talented and amazing Alonso Ramirez Ramos