I read somewhere that gender in modern Greek is fixed by the article that is used, so οδός is not masculine as we might expect, but feminine because the article is η and μίa
Um, that's kind of the wrong way around.
The gender is part of the noun, and the article has no choice but to follow the gender of the noun.
So οδός is feminine and that is why you need the articles η, μία. So you need feminine articles because the word is feminine, rather than the word being feminine because you use feminine articles.
Some words can even have multiple genders, e.g. σύζυγος "spouse": ο σύζυγος is "the husband" and η σύζυγος is "the wife".
That makes much more sense, and restores my faith in the 'logic' of Greek.
So, ιατρός can be a man or woman?
Ah, ο ιατρός and η ιατρός?
My lady doctor is Κυριάκη, but also on weekdays. :-)
It is also fixed by word ending because articles can be the same (for example οι is both feminine and masculine nominative plural). Nouns in -ος could be of my gender even in Ancient Greek.