Gamma World is not D&D.
In the first place, it’s not about reenacting the “zero to hero” character arc. It’s also not a funnel; it’s not about creating a deadly environment and then weeding out the weak, the slow, the lame, and the unlucky. The chaos of the Shadow Years has already accomplished that to a far greater extent than any D&D campaign!
The rule book puts it this way: “Player characters represent an elite with the desire, the initiative, and the ability to venture outside the boundaries of the village, town, or tribal lands. They are the pioneers, explorers, and tamers of the vast wilderness. It is they who will eventually bring order to the chaos of GAMMA WORLD and an end to the Black Years.”
In other words, you are not Merry and Pippin venturing outside of the bounds of the Shire for the first time. You’re Hiero from Sterling Lanier’s Hiero’s Journey. Therefore, rather than rolling 3d6 six times in order to generate your attributes like you would in Basic D&D, you roll 4d6 and drop the lowest. But note that unlike the way that AD&D is typically played you do not get to arrange then as you like. There are no dump stats under the original first edition Gamma World rules.
And unlike D&D characters that start with a single hit die and then play very, very carefully until they can (maybe) graduate to a new level where they might get a precious few additional hit points, Gamma World characters are much, much heartier. They start with a number of d6 hit dice equal to their already larger than average Constitution scores!
The class and level system that is the touchstone of every D&D and D&D-like games is simply obliterated. Your to-hit targets are a function of weapon class versus armor class with levels not really coming into things at all, really. Your saving throws against things like poison and radiation don’t go up according to a schedule based on your level. Your Constitution score determines that– just like your Mental Strength determines how well you’ll fare resisting mental attacks. Finally, there is no outline of what sort of followers and strongholds you’ll establish once you reach “name level” and so forth. You’re going to have to be content with whatever amount of followers your Charisma can pull down for you!
(Evidently the default milieu of the D&D is far more civilized and orderly in comparison.)
Incredibly, you don’t even gain additional hit points when you level up. Instead, you roll on a table of bonuses that result in either +1 to an attribute, +1 to to-hit rolls, or +1 to damage. If this results in your character gaining +1 to an attribute that is already at 18, you don’t gain any new bonuses at all!
Finally, Gamma World annihilates the premise of magic-users starting the game with only one spell and clerics beginning with no spells at all. Mutant characters typically start with several spell-like mutation abilities. Many of them are “always on” or else can be used many times a day.
For people that have spent copious amounts of time playing traditional D&D games, all of this is going to be a tremendous departure from business as usual. You’d almost wonder what the point of a game where you start out so tough and then gain relatively little as you level up. What do you even do?!
Again, I’ll point you back to the to the game’s on description of its player characters: “It is they who will eventually bring order to the chaos of GAMMA WORLD and an end to the Black Years.” Leveling up to gain more abilities and hit points is not what this game is about. Taking initiative to alter the status quo of the overall campaign state is. And that really is a thoroughly un-D&Dish proposition. To tackle that job, you don’t even have the usual benefit of spending 3d6x10 gold pieces on iron spiked, 50′ rope, and a ten foot pole. The equipment, weapons, and armor you’re going to need to tame the wilderness are out there. And figuring out how to use it can kill you!
I gotta say, I’ve seen many games that amount to little more than a straight up re-theming of either Basic D&D or Swords & Wizardry or some other retro-clone. But TSR circa 1978 really didn’t do that sort of thing. This is something that the OSR has lost in a very big way and it’s always struck me as odd. Granted, there is some utility in turning the most played rpg rules into something closer to a generic system covering many genres. But I really would have thought that designers today would take the same sort of risks and make the same sort of sweeping changes that James Ward did back in the day.
Mostly they don’t.