Possibly an American Bullfrog, Rana catesbeianus, or a Green Frog, Rana clamitans. Or something else, but those are the most common frogs that are consistent with the details visible in your picture.
More details on where it came from would help (what state was it found in and if it was caught outside). If possible, a closer picture wouldn't hurt either.
I'd probably avoid keeping it on that aquarium gravel, if it swallows a rock it could cause an impaction.