Thanks for writing this out - I just de-leeched 何月, hopefully for good.
I AM SO THANKFUL FOR THIS.
Here’s my mnemonic now.
がつ - “Gots” – NUMBER – You gotts to catch 'em all – and there are… Well, now there are 932 of them. But there’s a NUMBER OF them. You GOTTA catch 'em all, so you use GATSu with the number.
げつ - “Gets” – Not-A-Number – Do you get me?
This is extremely helpful!