I ran about 5 sessions of a Gamma World game using the BRP rules. If I did it again I would have expanded out the mutations to better reflect the gonzo and had a good go to table for making mutants. What I did have however was have the races tuned in pretty well.
Since BRP and Gamma World in its D&D incarnations used 3d6 stats I kept Pure Strain close to that with them as +3 con, int and APP with full use of all prewar gear and not being able to mutate by contact to low grade mutagens and radiation
Mutant humans lose the stat bonus but had a selection process for mutations I was continually tweaking to represent lightly mutated/lucky and heavily mutated random individuals. As well being able to use gear unless it had soultech DNA checking or mutations had altered them heavily enough to keep them from using gear due to antlers or whatever.
Mutant animals types were a huge revision based on a 6 point system to buy your species of real world animal based on its actual abilities. 1 point would be aquatic but not able to breath water, dangerous claws or hooves, a bite attack, +2 mov or a minor movement gimmick ability like a sticky tongue, prehensile tail/trunk or ability to glide but not fly or hide scales worth 2 natural armor. 2 points would be a major power like 2 extra arms or legs, fully aquatic and can breath underwater, wings that can fly, 4 points of armor or a huge maw bite attack for 2d6 w/ half damage bonus.
+6 points to any combination of dex, str and con with a further +4 at a +1 for -1 tradeoff with another physical stat with finally a +2 or -2 siz mod based on what animal they are.
This basically cut out the superfluous genotype charts from the old TSR version and how they were lumped in with the mutants for no flavor.
Sentiant plants were pretty close to the old TSR stats with a few modifications stuff like trees +2 str and natural armor but not against fire and fungi getting psionic resistance.
Worked in a bit of Espers and Nano attuned folks as if they were magic users due to how they manifest and control nanotech on a almost subconsious level.
Players did NOT dig the "junk is loot" aspect and it fell apart sadly enough