You can make your own gamerules

From the MC Wiki

In addition, new game rules may be defined and set (by providing a value) or queried (by not providing a value). For example,

gamerule MyNewRule 10

defines a new game rule called MyNewRule and sets its value to 10. Attempting to query a game rule which has not yet been set will cause the command to fail (a change from the previous behaviour of always succeeding).

Confirmed. The gamerule goes into the level.dat file, which is why it is auto-completable.

Working as intended.

This can't be working as intended. Sonic's source says:

In addition, new game rules may be defined and set (by providing a value) or queried (by not providing a value). For example,

gamerule MyNewRule 10

defines a new game rule called MyNewRule and sets its value to 10. Attempting to query a game rule which has not yet been set will cause the command to fail (a change from the previous behaviour of always succeeding).

http://minecraft.gamepedia.com/Commands#gamerule

The bolded part is actually not true. As Sam described in his report, the command does not fail. It succeeds and can activate a comparator. Output of the command reads "Gamerule has been updated".

Please reopen this report. False gamerules should not succeed.

No. Tthis ticket "You can make your own gamerules" is working as intended.
If you've found a bug in the behaviour of custom game rules, create a new ticket.