If you want some decent reading material that might give some ideas for a Gamma World that is less on the "wahoo" side, check out "Hiero's Journey", which is basically Gamma World in Canada. The protagonist has some potent mental powers, rides a semi-intelligent moose, and is friends with a sentient, telepathic bear. Mutant villains include mutant mentalists, rat-folk, monkey-folk, giant frogs, and a few others.
If you can accept the mental powers stuff (which CAN get a bit wild and wahoo), the setting is pretty low-key. No fire-breathing monsters, no laser eyebeams, no bunnies turning metal into rubber. I guess I'd say many of the standard GW mental mutations would be present but few truly unrealistic physical ones.