The (B)ig (G)old (B)ook for Chaosium's BRP is a decent touchstone for creating your own specific homebrew game world. The big advantage is the system is remarkably modular, allowing the Game Master to pick and choose levels of complexity, and the systems built in internal consistency. When coming up with your own game universe and setting it sure helps to be able to rapidly come up with a rules judgement without having to search through rule books for special cases.
The other place where BRP shines is character creation. You can follow the step by step character creation instructions and quickly customize the mechanics to develop player characters tailored to your desired setting. The most useful feature is the "Guide to Creating A Character" flowchart. On two pages there are 10 steps clearly illustrating how to build your PC. While all these steps are detailed within the pages like you would find in any other RPG rulebook, this flowchart allows you to readily apprehend the overall process. I find this pictorial representation allows me to make design choices immideatly as I craft a brand new game. I could literally sit at a table with players and ask what kind of game do you want to play? If they answer 1920's gangsters good to go. Ruthless pirates on the Spanish Main game on in twenty minutes. If I have sourcebooks from other games on my shelf for the desired genre I have ready made setting material at my fingertips.
To keep things simple I took a stripped down approach to creating a Gamma World character. Specifically with skills. 250 point for skills. That's it. For powers there will be Mutations, of course. The guidelines for major and minor mutations will be taken. Four slots total, a major mutation will take 2, minor 1. Declare your selection and roll away on random tables!
The table of contents can give you an idea how versatile and organized the Chaosium approach is: Characters (covering basic attributes), Skills, Powers (further broken down into Magic, Mutations, Psychic Abilities, Sorcery, and Super Powers), System, Combat, and Spot Rules. The only thing left is your full on imagination.
But being a generic RPG rulebook there are going to be gaps. Gaps and holes. A common complaint against BRP is that it doesn't do Super Hero roleplaying well. That the normal to heroic human-centric baseline breaks down when designing super powers. By extension this critique can be applied to Sci-Fi, specifically vehicles. I think this is a legit complaint. The BGB even cops to this, and I quote; "a comprehensive listing of all vehicles or even most major types of vehicles would dominate this rulebook." Reading between the lines you realize that coming up with comprehensive vehicle rules and vehicle combat is an entirely new subsystem which needs to drafted for technologically advanced gaming worlds.
The other gap, and one much more easily managed, is skill lists specific for the genre being played. The BRP character sheet lists all the skills common to the system as well as blank lines to add additional skills as needed for the specific game. What this creates is kind of a mess which players and Game Masters have to sort out during character creation. Short of drafting a custom character sheet for each genre I recommend a Game Master provide a Complete Skill List to the players for the game. For example; if I was going to run a "Gamma World" inspired BRP game my complete skill list would look like this.
Gamma World Skills – Complete Listing
COMMUNICATION
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BARGAIN/BARTER
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COMMAND
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DISGUISE
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FAST TALK
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LANGUAGE, OWN
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LANGUAGE, OTHER
MANIPULATION
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CRAFT: JURY RIG
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DEMOLITION: EXPLOSIVES
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FINE MANIPULATION: PICK LOCKS, MECHANICAL
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FINE MANIPULATION: PICK LOCKS, ELECTRONIC
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FINE MANIPULATION: DISARM/SET TRAPS
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REPAIR: PRIMITIVE
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REPAIR: COMPLEX
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SLEIGHT OF HAND
MENTAL
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APPRAISE
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FIRST AID
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GAMING
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KNOWLEDGE SKILL: THE ANCIENTS
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KNOWLEDGE SKILL: SURVIVAL
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MEDICINE
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SCIENCE: THE ANCIENTS
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TECHNICAL SKILL: COMPUTERS
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TECHNICAL SKILL: DRIVE
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TECHNICAL SKILL: PILOTING
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TECHNICAL SKILL: POWER ARMOR
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TECHNICAL SKILL: ROBOTICS
PERCEPTION
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LISTEN
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SCAVENGING
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SPOT HIDDEN
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TRACK
PHYSICAL
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ATHLETICS (covers Jump, Swim, Throw, etc.)
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CLIMB
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DODGE
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HIDE
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RIDE
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STEALTH
COMBAT
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ENERGY WEAPONS
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SLUG THROWERS
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MELEE WEAPONS
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RANGED WEAPONS
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BRAWL
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GRAPPLE
For Mutations you have rules and examples provided in the BGB, and Psychic Abilities will most likely come into play. But the list is not altogether long. It has gaps.
The Metamorphica, Being a Very Large Collection of System-Agnostic Random Mutation Tables by Johnstone Metzger is indispensable for building out your mutations list for a gonzo-tinged game of Gamma World. The BGB makes a distinction between Major and Minor mutations so random rolls on the the
Metamorphica tables will take some work with the Game Master. I decided my Mutant Humanoid I'm rolling up will have one Major and two Minor Mutations. I rolled Hallucinations, Reverse Pedalism, and Bad Breath. Okay, so my Major is technically an affliction. The character would be severely plagued with Hallucinations. Not very gameable. So I will turn this into the psychic ability Emotional Control. The effect, and power of this ability is based on the Power attribute. As a Major mutation I decide the character's Power attribute is
doubled when using this ability. Reverse Pedalism just means he has four legs. As a minor mutation I will increase his max movement by 50% then say double if it was a major mutation. No Gamma World character is complete without some type of negative mutation and this guy has got Bad Breath. Like nasty, gut wrenching bad breath. Why not say if he successfully Grapples an opponent they can paralyze them if the victim fails a resistance roll.
So there you have it, a Hallucinating projecting, four-legged, foul-mouthed mutant humanoid striding the radioactive wastes!