This is a Grey Tree Frog.
The gray treefrog (Hyla versicolor) is a species of small arboreal frog native to much of the eastern United States and southeastern Canada. (Source: Wikipedia, Gray_tree_frog, CC BY-SA 3.0)
They can change colors to match its surroundings to help protect itself from predators. We though it was a piece of gravel in the gardens, so just be aware when cleaning up your gardens in the springtime. Each and every critter has a important place in our eco system.