As far as I know there isn’t any rule to learn. Grammatical gender is a wild mixture of several things, sometimes it has something to do with the ending of words and sometimes with attributes of the things, if it has like agency, is an inanimate object, or is an abstract concept. Sometimes it’s completely arbitrary and sometimes there are rules to it like with group of people. But there is no way of telling, you got to memorize it. In any way, grammatical gender has nothing to do with biological gender. And I’m pretty sure that’s not it’s origin. Though, we try to link it to biological gender in case of people. But even that has exceptions, and it doesn’t really work with group of people etc.