If the subject is feminine, why should I describe it with a masculine word
Would not be better to write ela é uma gata instead of ela é um gato? Is the subject a non binary gender person?
Look at the comment two above yours if you got docked for "gato" Then you might have used "uma" which means if would be looking for "gata" after the "mua" and it would only tell you that "gato" was wrong cause just that word was in the wrong gender.