To create a gamerule, build and register it using the GameRuleBuilder of the Fabric API:

// Create and register a boolean gamerule, enabled by default
private static final Identifier        GAMERULE_IDENTIFIER     = Identifier.of("custom_mod_name","gamerule_name");
public  static final GameRule<Boolean> CUSTOM_BOOLEAN_GAMERULE = GameRuleBuilder
    .forBoolean(true) //default value declaration
    .category(GameRuleCategory.MISC)
    .buildAndRegister(GAMERULE_IDENTIFIER)
;

To get access to the value of a gamerule, use a server-world:

serverWorld.getGameRules().getValue(CUSTOM_GAMERULE);

The registered gamerule shows up untranslated in the gamerule list, which can be opened in the world creation screen. To translate the name, add an entry in the lang-file (see lang for more info):

{
  "gamerule.custom_mod_name.gamerule_name": "Gamerule Name"
}