It is ostensibly a kids show that was aired on Disney XD but is so well written it should not kept just for kids!
It works on both levels really, but slightly differently than most - less knowing winks at the adults, more a kind of understanding of human nature in the way the characters are written, it really is superb. The villains are great, and there’s an overall plot that threads through the whole thing that builds to a brilliant climax.
I'd say it's mostly an early teens show (hence the age of the characters), but written with a lot of respect for the intelligence of its audience. And it's very well written.
But like Rick and Morty, Community etc., it's also got a lot for adults as a genre send-up show. There are a lot of fantasy/sci-fi tropes - not as inventive in quite the same way as Rick and Morty, which comes up with its own worlds and monsters.
Whereas Rick and Morty is constantly teasing the audience about being a nihilistic show via Rick, Gravity Falls pretty much always wears its heart on its sleeve. There's a sort of similar character in temperament and attitude to Rick, but he's slightly less central than Rick, and he's more of a lovable asshole type, whereas Rick conforms more to a high functioning sociopath archetype.
One show is snarky and the other is basically very sincere and heartfelt. Rick and Morty is Hitchhikers while Gravity Falls is more like a scifi/fantasy Stand By Me (with more likeable adult characters).
