Gamma radiation, the form of ionising radiation that's an electromagnetic wave, is so small it doesn't really reflect off things, it'll pass straight through a mirror for instance, or y'know, a thin sheet of lead. Better to block it by giving it something harmless that can intercept it and get ionised that's not DNA. Though in this case it appears that it's more that melanin can mop up free radicals and reduce DNA damage that way. Basically, you can't realistically block gamma radiation with frog skin, but you can stuff it full of compounds that mop up the damaging byproduct of being hit with it.