WK says that the vocabulary and kanji for 門 (both level 16) are exactly the same. But it accepts gates (plural) as a meaning for the kanji, but only gate (singular) for the vocabulary.
Add synonym and move on.
… after trying to improve the experience for others.
For suggestions for improvements, it’s best to email the team for future updates. You’re right in that it’s best to have both as acceptable answers.
That’s a good idea in general, but I think it’s a lost cause. You’d have to keep on emailing them about every item where a synonym would be useful or something else needs changing.
Imho the kanji shouldn’t accept “gates” as a meaning - the kanji represents the concept of a gate. Only If it’s a standalone vocabulary item it’s “gate” or “gates”. From a broader perspective it doesn’t matter, but it fits the design of WaniKani better.
But, you can just @ them and they usually reply and accept the idea.
I’ve made the change and ‘gates’ is now accepted as a meaning for 門.