Minecraft wiki entry for gamerule, associated text: Specifies the value to set the game rule to. If not specified, queries the value of the game rule instead of setting it.
Only true or false specified for predefined game rules can affect gameplay, except in the case of maxEntityCramming, randomTickSpeed, spawnRadius, playersSleepingPercentage, functionCommandLimit and maxCommandChainLength where any integer affects (-2147483648~2147483647) gameplay; see Game rule.