Gravity Falls has a few references to classic Universal monsters that you may have previously missed. The animated series is a kid-friendly show, but creator Alex Hirsch also made sure that older audiences could enjoy it. While many have focused on how Gravity Falls could connect to Rick and Morty, the show worked in several allusions to iconic horror movies and characters.
The monsters and mystical elements work with Gravity Falls thanks to the unique setting of the series. Dipper and Mabel Pines only visit Gravity Falls for one summer but soon discover that there is more going on than they believed. They learn about paranormal and supernatural creatures lurking around the town and get wrapped up in several adventures as they try to protect it.
The premise for Gravity Falls allows it to reference some of the biggest horror films ever, but the series has an affinity for one horror franchise specifically: the Universal Monsters. These characters created the first shared universe back in the early 1900s with the likes of Frankenstein, the Mummy, and more existing together. Gravity Falls doesn't directly adapt those characters but, as explored in the Screen Rant video featured at the top of this post, it does reference two Universal Monsters, Gill-Man, and the Invisible Man.
Both of these Universal Monsters references in Gravity Falls are easy to spot if viewers are familiar with the iconic creatures and know where to look. The appearance of Gill-Man is one of the earliest references viewers will catch in the series thanks to Stan Pine's introduction. When he first graces the screen, Stan is wearing a mask that resembles the Gill-Man, the titular character in The Creature of the Black Lagoon. Stan is also at the heart of Gravity Falls' other Universal Monsters reference, so he may be a big fan of the old films. As part of the Mystery Shack, he has the Invisible Man "on display" for attendees to see.
The brief attention that the Universal Monsters receive in Gravity Falls may be better than how they've been treated most recently. Universal Studios tried and failed to launch a new cinematic universe for these characters, but they aren't staying off of the big screen for long. Blumhouse and director Leigh Whannell are currently working on The Invisible Man, with Oliver Jackson-Cohen cast to play the title character. A new Creature of the Black Lagoon movie is not currently in active development, so a possible Easter egg of him in The Mummy may be the closest fans get to see the Gill-Man on the big screen again for the time being.