Cranberries contain a massive amount of natural pectins. They will gel all on their own, which is why you can basically make cranberry sauce out of filling.
Alton BrownYou need to either create a slurry in a cold liquid, which also works with cornstarch, or you've got to do your gravy in a very wide pan and kind of scatter the flour over the top and then very quickly whisk it in.
Alton BrownMy advice: write down everything you eat. It's amazing what that "self honesty" can do for you. (Do you really want to have to confess that doughnut? I thought not.)
Alton Brown