Yes, this has been throwing me into so much confusion lately. Just when I thought I have a handle on when to use one – my WaniKani proves I don’t actually know 🙂
After a bit of searching though, I found a great post that may help you get this squared away once and for all. In Japanese you count the months numerically, with がつ appended. For example, June would be roku-gatsu (六月) – and September would be ku-gatsu (九月). Every time you have a numbered month, you use ‘gatsu’, you may then say well, when you ask what month (nan-gatsu) there is no month numeral prefix! Whats going on there then??
Well, the question 何月 is actually indirectly referring to a numeral value e.g. 6th month, or June (roku). So thats why it uses gat-su. Easy!
Now the next one for me to get a better understanding of is when to use にん or じん!