Really, I'm just fleshing out some of the stuff that's been previously mentioned in the comments above, but here are a few things to consider when discussing Disney's possible involvement with a potential cancellation.
1) Awesome ratings! Gravity Falls is arguably Disney XD's highest rated animated show, averaging around 1M viewers per episode (that's twice as high as Ultimate Spider-Man with its 500K). That's not even including digital downloads and On Demand streams. It doesn't necessarily mean it's twice as profitable, though, so keep that in mind.
2) The move to Disney XD. Now, this part's pretty interesting. After all, the show was doing great on the Disney Channel. Like, really great; averaging about 3.2 million viewers. That's even higher than Phineas and Ferb! In other words, the move was likely meant to promote Disney XD rather than the show itself; or, as I'd like to call it, a "dick move". But, if this theory is correct then, that would mean that Disney had quite a lot of faith in Gravity Falls. At least back in 2014.
3) The air schedule. Yes, yes, we all know that it sucks. Now, don't ask me how it works because, as far as I'm concerned, it makes no sense, but this is supposedly a proven method that, again, supposedly boosts ratings AND is something that's generally reserved for shows with lots of dedicated fans. But the important thing to take away from this is that Disney considered Gravity Falls to be a very important show. Or, you know, they could just be f*cking with us.
4) Upcoming merch. Disney has suddenly decided to up the production of merch, most of which is scheduled for next year. If the show was ending, why do so after the fact, when people are starting to lose interest?
5) Cancellation or delayed renewal? Yeah, I don't buy it. I don't think Disney flat out cancelled the show. I mean, I guess there could have some major production issues that we are unaware of. Budget cuts, maybe? Judging from the animation, Gravity Falls never struck me as a show with a load of cash behind it. They always seem to save the good stuff for important episodes like the Pilot and Not What He Seems. Not that there's anything wrong with that, the rest of the animation isn't bad or anything, but it does kind of suggest that they're at least somewhat tight on money. So that could be an issue. Just speculating, though. It really shouldn't be a problem and if it is, well... Disney be damned.
Is Gravity Falls coming to an end? Damn if I know. There's plenty of circumstantial evidence suggesting either. But if this really is it, I don't think Disney's the cause.