The syntax for the /gamerule command is:

/gamerule <rule> [value]

(This syntax can be seen through the /help command.)

But, since typing only /gamerule will return all available gamerules and not any error message, the <rule> field isn't required, so it should be:

/gamerule [rule] [value]

The fix for this bug is simple and only involves changing a string on the language files.