On the description page it says:
“The most common reading for this word has the on’yomi reading for 毎 plus the kun’yomi reading for 月. That makes it まいつき. But it can be read まいげつ using both on’yomi readings too. You can use either!”
But it doesn’t accept まいがつ as a valid reading, so either it does accept “both on’yomi readings” or it doesn’t.
Also why is there change from がつ to げつ in 毎月 but no change from にち to じつ in 毎日? I was under the impression the change to second form happens when we treat both Kanji as a single word, like “monthly” instead of “every month”.