To create a gamerule, register it using the GameRuleRegistry of the Fabric API:

// Create and register a boolean game rule with the name "shouldPigsFly" which is true by default.
public static final GameRules.Key<GameRules.BooleanRule> SHOULD_PIGS_FLY =
GameRuleRegistry.register("shouldPigsFly", Category.MOBS, GameRuleFactory.createBooleanRule(true));

To get access to the value of a game rule you need to get the server world:

world.getGameRules().getBoolean(SHOULD_PIGS_FLY)

The registered gamerule now shows up untranslated in the game rule list, which can be opened in the world creation screen. To translate the name, add a gamerule.gameruleName entry (see lang for more info):

{
  "gamerule.shouldPigsFly": "Pigs should fly"
}