Eastern Gray Tree Frog

Like I said in my previous comment, they are visually indistinguishable. Both species are highly variable, color and pattern cannot tell them apart. The only way to tell them apart definitively is by the number of their chromosomes. Technically also by their call, but only if they are at the same temperature, and you have no way of knowing if they are at the same temperature. You have both species in range, and there is no way to tell which species this is. You can call it a Gray, but you also need to mention it can also be the Cope's Gray but there is no way to tell. And please add the scientific name to complete your spotting.

Also, it's heavily forested where I found the frog.

I believe it to be an eastern gray tree frog, after looking at the Minnesota DNR website. Cope's tree frogs evidently don't tend to have as much marking on their backs.

These two species are visually indistinguishable and are both in your area, so at this point it is impossible to tell which one it is.