There actually is some evidence that Gravity Falls and Rick & Morty could be a part of the same narrative.
First, there are two characters that some believe may actually be the same guy. In a since deleted 2017 Reddit post (via Decider), a fan put forth the theory that — pointing out physical similarities, and the fact that both act as somewhat omniscient in relation to the rest of the characters – Rick & Morty's Mr. Poopybutthole and Bill the Cipher of Gravity Falls are actually the same character.
Second, there are some obvious similarities between Rick and Stan, including the fact that both have been fugitives at one time or another, and both have traveled across dimensions. Some fans even wonder if Stan was along for the ride back when Rick used to get up to shenanigans with characters like Squaunchy and Birdperson.
Third and finally, the creators of both shows did connect their universes. In the season 1 Rick & Morty episode "Close Rick-Counters of the Rick Kind," while Rick and his grandson flee a pack of alternate universe Ricks, the scientist uses his portal gun to open lots of interdimensionals portal to lose his pursuers, and out of one of these portals flies a pen, a notebook, and a coffee mug. Six months later, in the season 2 Gravity Falls episode "Society of the Blind Eye," Stan loses these three items as they fly into an interdimensional portal.
So, will we see an official Gravity Falls/Rick & Morty crossover one day? Considering the adult content of the latter show, and Disney's avoidance of that material, don't hold your breath, but you never know. Maybe Meeseeks could make it happen.
