Gamma World à la Stephen

A love letter to classic Gamma World by means of a Classic Traveller-esque 2d6 ruleset. [Cheat Sheet]

Table of Contents

  1. Throws
  2. Combat
  3. Powers and Mutations
  4. Inventory
  5. Experience
  6. Character Creation
  7. Referee Tools

1. Throws (Core Mechanic)

When you perform a risky task, Throw (roll and sum) two six-sided dice (2d6). All dice are rolled in the open.

After rolling, add any applicable Bonus or Penalty modifiers to the sum of the roll before comparing the result to the target. A bonus always adds +1. A penalty always imposes -1.

To successfully achieve your aim, meet or beat the relevant Target. In most cases, the target is 8+.

Bonuses and Penalties

Bonuses and Penalties accrue from various sources. Skills apply to any relevant throw, while Traits apply to any relevent throw outside of combat. There are assorted Situational Modifiers that may also apply to certain rolls.


Characters are rated in each skill as either Trained (bonus of +1), Background (+0), or Untrained (penalty of -1). When a Skill applies to a character's action, add that skill's Rating to the roll.

Characters normally begin with two skills from the Adventuring Skills List rated Trained (+1) and two rated Background (+0). Each character also begins with one skill from the Combat Skills List rated as trained (+1) and all others as background (+0). (All player characters possess adequate background knowledge with all combat skills.)

The Referee will curate the Skill List for the given setting, e.g. including a Spacefaring skill for a sci-fi game or a Scavenge skill for a post-apocalyptic setting.

Example: Whipsaw is trained in Hand Weapons. He also possesses professional-grade skills (+1) in Athletics and Scavenge and substantial practice (+0) with Bushcraft and Stealth.

Personal Characteristics (Traits)

There are seven Traits: strong, tough, agile, convincing, perceptive, clever, and disciplined. Characters normally start with two bonus traits (+1s) and one penalty ("not very") trait (-1).

Example: Whipsaw is strong and tough (+1), but not very clever (-1).

Situational Modifiers

Add a bonus (+1) to your roll when you benefit from an overall situational advantage, based on such factors as exceptional tools, a power that would boost your natural capabilities, thorough preparation, surplus help, or other benefits of fictional positioning.

Inversely, impose a penalty (-1) to your roll when you suffer from an overall situational disadvantage, based on factors like inadequate tools, a hindering mutation, insufficient preparation, and so forth.

Impose an additional penalty (-1) when you confront a particularly difficult situation, especially when facing serious pressure (like limited time, impending danger, or ongoing combat) or high complexity.

Group Tests

For Group Tests, i.e. tasks involving the entire crew: either nominate one character to roll for the team (in situations when expert guidance would be decisive), or else everyone throws against the target, with success if half+ of the crew passes. In both cases, the entire crew shares the consequences of success or failure.


Gain one Boon whenever you roll natural doubles on any throw. Boons may be spent at any time to grant another player's character a reroll on a failed throw (except a Down and Out test).

2. Combat

When you attempt to harm an opponent in combat, throw 7+.

Attack and Defense

Bonuses apply to your Attack when you:

The throw is also modified by the target's Defenses. Penalties apply if the target:


If the throw is successful (7+), you harm your opponent: deal points of Damage equal to the lower of the two numbers showing on your thrown dice plus any damage modifiers:

1Explosives deal double damage to organisms at point blank range and the listed damage in an area. A save is usually appropriate for partial effect.

Life and Death

Starting characters have eight Hit Points (HP), which measure characters' abstract staying power in a fight (stamina, focus, pain threshold, raw luck, etc.). Damage reduces Hit Points on a one-for-one basis.

When reduced to zero HP, you are Down and Out. An ally may attempt to revive you with a first aid throw of 6+ (disciplined and medicine apply; failure inflicts a hit). If you receive prompt medical aid, throw 2d6 without the usual modifiers: If you roll doubles, you die gruesomely. Otherwise, subtract one from the roll for each time you were hit (for any amount of damage) after going down. If the result is 7+, you avoid permanently losing one point of maximum HP from lingering injury and you immediately gain one XP.

To regain lost HP, you must eat and rest. A Short Rest takes an hour and allows any characters who are holding Rations to regain 1d6 lost HP. An overnight Long Rest, in contrast, allows any characters who are holding Camping Gear and Rations to restore all lost HP, refresh any exhausted Powers, and gain a Boon if they currently have none. Characters do not require held supplies to benefit from resting while in a safe haven.

3. Powers and Mutations

Characters receive Powers from mutations, augmentations, or Ancient tech. Powers work in different ways: some provide a bonus or reroll to certain throws, some provide new capabilities, some enhance an existing ability, and some are just rare and powerful tools.

A Descriptor grants you a power as negotiated with the Referee. Moreover, descriptors and Types sometimes provide a situational advantage (+1) or disadvantage (-1) to appropriate throws that go beyond providing a novel power and instead improve an action that a character could already take (e.g. a wolf-folk tracker searching for a scent).

Regardless of their source, all powers—including descriptors—occupy one Inventory Slot.

Player characters may gain Mutations in play, such as when they fail a saving throw against strong radiation exposure or choose to expose themselves to powerful mutagenic xenotechnology. When a random mutation is gained, the player may choose to roll on either the Descriptors (Chart B) or Animal-folk (Chart C) tables.

4. Inventory

Each character possesses ten Inventory Slots for gear and powers. Each significant item or package normally occupies one Slot, though particularly bulky or unwieldy items (e.g. armor) require an additional Slot. All powers, including Descriptors gained through a Mutation or Augmentation, also occupy a slot.

Certain consumable and rechargeable items must be checked for Depletion after every scene in which they are used. Throw against the listed Depletion (Dep) target. On failure, the item is exhaused and must either be discarded, recharged, or (if applicable) repaired.

5. Experience

Gain one Experience Point (XP) whenever you meet the trigger specified by your Class Archetype.

In addition, each player gains one XP at the end of each session, and then the character with the fewest XP gains another XP.

You may spend XP at any time to purchase an Advance.

Cost (XP) Advancement
2 +1 HP1
+Inventory Slot2
4 +Mutation
+Follower Slot3
6 +Adventuring Skill
+Combat Skill
8 +Trait4

1 To a maximum of 20 HP.

2 To a maximum of 20 Slots.

3 A Follower begins with 2 HP and one skill. If a Follower dies or departs, they are automatically replaced at the beginning of the following session. You may choose to have the +HP, +Slot, or +Skill advances improve one of your Followers rather than your primary character.

4 May only be selected twice.

6. Character Creation

When you create a new character:

  1. Choose two Traits as strengths and one as a weakness (not very.)
  2. Choose one Combat Skill as trained (+1). The remainder are background skills (+0).
  3. Choose two Adventuring Skills as trained (+1) and two as background skills (+0). The remainder are untrained (-1).
  4. Start with eight maximum Hit Points and one Boon.
  5. Each character possesses ten Inventory Slots for gear and powers (each significant item, package, or power takes one slot).
  6. Choose an Origin and record its changes to your statistics.
  7. Choose a Class Archetype. Gain its abilities and your choices from its starting gear.


Skill Lists

Combat Skills

Adventuring Skills


Pure Strain Human


Class Archetypes


  1. Martial Expertise: Start with +4 HP and training in a second Combat Skill of your choice.
  2. Momentum: Gain one XP whenever you recover a powerful Ancient Weapon or personally best a well-matched foe in pitched combat.


  1. Preparation: Start with two additional trained (+1 bonus) Adventuring Skills of your choice and gain +2 Inventory Slots.
  2. Problem Solver: Gain one XP whenever you recover a great treasure or use your wits and ingenuity to overcome an intractable obstacle.


  1. Tribal: Gain two human Followers (2 HP, one skill, one Slot; may be automatically replaced the following session if they die)
  2. Personnel: Gain one XP whenever your patron, allies, or tribe acknowledge your service or when your crew plays a crucial role in overcoming an obstacle.


  1. Unstable: Whenever you gain a random Mutation or Augmentation, roll twice and choose one to gain. Gain a new Descriptor (Chart B) now.
  2. Delver: Gain one XP whenever you experiment with Ancient technology, machinery, or installations; overcome an Ancient-tech puzzle or trap; or gain a new Mutation or Augmentation.

7. Referee Tools

Reaction Rolls

  1. Armed and Hostile
  2. Panicked/Alarmed
  3. Cautious
  4. Indisposed (drink, injury, sleeping)
  5. Diplomatic (possibly treacherous)
  6. Friendly

Morale (Save vs Rout)

Throw when NPCs go, "Oh shit!"

Throw 7+ Bonus organized, fearless Penalty disorganized, leaderless

Automatically fail when NPCs go, "Oh fuck!"


The Referee starts each session with no Doom. Whenever the Referee rolls doubles, she gains a point of Doom. Doom may be spent on NPCs just like Boons may be spent by players (to reroll a failed throw). All accumulated points of Doom are lost at the end of the session.

When conflicts with bosses occur (or at other declared times, such as in a profane temple or irradiated wasteland), the Referee immediately gains 1d6 Doom.

Adversary Statistics

HP 4/8/12/18/24-30   Attacks -1/0/+1/+2

HP ranges from Puny (4) to Human-Like (8) to Soldier (12) to Brute (18) to Monstrous (24-30).

Attack ranges from Weak (-1) to Average (+0) to Trained (+1) to Lethal (+2). Ancient-tech used by adversaries will increase the attack score according to the gear's details.

Stephen Parkin, © 2020