Back in the earlier days of Minecraft (think of Minecraft 1.8), this was not possible. But when I forget a gamerule, I just start typing a gamerule and then just hope it is the good one... I wanted to change the gamerule 'doDaylightCircle', but as you can see, I typed in '/gamerule daylightCircle false'. The strange thing is is that this "false" gamerule (daylightCircle) has been added to the gamerule list... I wanted to verify that this wasn't a one-time bug, so I tried another "false" gamerule, this time it was hahaha, and this was just the same story.

#1 Enter the command '/gamerule daylightCircle false'.
#2 There will come out: Gamerule daylightCircle has been changed to false.
#3 Enter the command '/gamerule'.
#4 Check in the list if there is the gamerule daylightCircle now (I think there is).

(Go on to #5 if you checked #4)

#5 To verify this: Enter the command '/gamerule hahaha false'.
#6 There will come out: Gamerule hahaha has been changed to false.
#7 Enter the command '/gamerule'.
#8 Check in the list if there is the gamerule hahaha now.

If you checked #8, this is definitly a bug.

Hope you can do something...