When to Use Gatsu (がつ)vs Getsu (げつ)

So, through the past week while going through WaniKani, I've been totally screwing up the difference between these two pronunciation and when to use them. So I decided to go the extra mile and really figure out why my brain is totally not capturing these two in the ways they are being used (at my current level in WaniKani lvl 6). So here it is the best way that I can explain it after doing my research and learning why I keep getting them mixed up. Maybe this will help you as well? The best way that I found to remember when to use GATSU vs GETSU is by looking at the word GATSU as a way of counting through the months of the year. Example: The months of the year in Japanese are pronounced by the numbers 1 through 12, so juugatsu is October, nigatsu is February etc. But if you want to go through a time frame/duration through months then it changes a bit and goes with GETSU. The counter is literally -ka getsu when using it for duration, you'll add a -kka getsu for duration of 1, 6, 8 and 10 months.
Example:
1 month - ikkagetsu 2 months - nikagetsu -kka -- 5 months - gokagetsu 8 months - hakkagetsu -kka -- 3 months - sankagetsu 10 months - jukkagetsu -kka

This is expressing a duration of time remember that A few more examples: Last Month, This Month and Next Month and Every Month Sengetsu, Kongetsu, Raigetsu, Maigetsu It's best to view these two as:
GETSU = ACTIVE: Movement of Time - Maigetsu/Ikkagetsu GATSU = STATIC: No Movement / Statement - Nangatsu/Ichigatsu

as I keep coming across more of gatsu vs getsu I'll keep refining this pattern. I hope this helps you as it has helped me learn a bit more about which ones which. -Nihonscope