Gridlock 2.0 is a complete rewrite. In iOS 6, Apple changed how icon movement works, and it made Gridlock a lot harder to update.
Because of that, if you’re upgrading from the original Gridlock, it’s not free. I don’t like charging twice, but this time the amount of work required it.
You do get a discount, though, and I will try and make future updates free if I can. And if you’re on iOS 5.1 or below, the old version still works.