I teach middle school and I love to show occasional episodes/clips of Gravity Falls for rewards, and even use if for lessons on plot and character.
Any other show as good as this for kids this age? Only Avatar comes to my mind along with its spinoff Korra.
I like the comedy side of Gravity Falls more, though. It’s very light and fun for kids without insulting their intelligence. The kids feel realistic as well. Anything else like it?
I’ll just point out that Soos is the real winner of the story.
At the very end, you see him running the store, wearing the hat and with his girlfriend running the register. 
Avatar, really. Nothing else comes close, in my opinion.
I know a lot of people love Steven Universe, though. It just didn’t do it for either my kid or me somehow. But as I said, lots of others love it to bits.
For adults, there is Rick and Morty, which is like Gravity Falls’ evil alter ego. 
Definitely not kid-suitable, though.
“Ned’s Declassified School Survival Guide.” It’s no longer in production and it’s available on iTunes, but I think it might fit the bill.
There’s the show I always confuse with Gravity Falls, Granite Flats.
Adventure Time?
Phineus and Ferb?
(I’ve seen nither, but heard good things about them)
Brian
Check out Over the Garden Wall. Definitely darker than Gravity Falls and only a mini-series, but it’s also well-written and beautifully drawn.
I enjoy Phineas & Ferb.
The Star Wars episode was awesome. It runs in parallel to the movie, so you actually see the movie characters and parts of scenes in the background. That and Isabella has a rivalry with Han Solo.
I don’t know- Gravity Falls could get really, really dark…
Bill Cypher: “I have some children I need to turn into corpses!”
Nope, not dark. 
While I haven’t watched them, I’ve heard many good things about Star Vs. the Forces of Evil and Wander Over Yonder. Although, both are more on the silly side of the spectrum.