GRAVITY FALLS HAS NOT BEEN CANCELLED
Yes, it is on hiatus. However, it has been confirmed that 21 episodes will be shown for season 1. Season 1 is also going to end on a cliffhanger, which means Disney Channel is basically morally obligated to continue on to season 2.
Besides, Gravity Falls is currently Disney Channel’s best-reviewed show (getting even higher reviews than Phineas and Ferb). Double Dipper’s premiere had the highest Nielsen ratings for any Disney show for 6 months. It is currently syndicated in thirty different countries. And it has a fan following to rival that of MLP:FIM.
In short, the Disney execs would have had to have gone through barrels of Smile Dip in order to be crazy enough to cancel Gravity Falls.
In the meantime, show your support. Watch Gravity Falls on its original network to boost up those ratings. Buy the episodes from amazon. Like GF on Facebook. Show them you care.
And last but not least, remember, in Gravity Falls, there is no one you can trust.