Exactly what it stands for. 7 and up.
TV-Y is for small children.
TV-Y7 is for 7 and up.
TV-Y7-FV stands for extra violence, but it should be fine if the child in question can understand the difference between fantasy and reality (and I had no issue doing so as a kid).
Some shows on Cartoon Network are TV-PG, which means parental guidance is needed.
But keep in mind that all of this is subjective. It depends on the child. When I was 7 and 8, many violent cartoons did not phase me. I stayed up all night watching shows like Gargoyles, which is honestly quite violent by todays standards when it comes to cartoons.
You know the little kids at your house better than we do; if you believe Gravity Falls is too much for them, then by all means don’t let them watch it. But the show’s rating confirms that it’s suitable to children 7 and up, even if there are definitely mature themes in the show.