Adventure Time is good if you like the mix of comedy and, well, adventure that Gravity Falls has. Bob's Burgers has a similar sense of humor. Also, Kristen Schaal voice acts a main character in GF and BB. read more
This is the answer I also gave for a similar question "What are some good cartoons like Gravity Falls"-Steven Universe. Almost all Gravity Falls fans love this show too, and it's just amazing. It's still running.-Over the Garden Wall. A mini series, I think only seven episodes. But it is so captivating and really draws you in. It's truly unique. read more
- Adventure Time, and Stars vs The forces of Evil. I personally don't care for either of these, but a lot of people who liked the above shows seem to enjoy these two a lot, so you should try them too. Don't know much about them, but they're similar to Gravity Falls and Steven Universe. read more