I have the same issue since 2-3 days, after having used a Google account via Apple Mail for several years.
Changing the security settings in my Google account helps - but of course that makes my account more vulnerable to attacks, so I don't really like it.
The main question is: Why is Google seeing Apple Mail as a "less secure" app?
Is that just a mistake on their side - or do they have a good reason for this? I hope that someone at Apple is working with Google on resolving this, because whether for a reason or not, this label is not exactly good marketing for Apple.