My recommendation is to turn off less secure apps and enable two-factor auth. (The app password is a one-time use password that can be voided if you suspect it is compromised, without affecting your “real” password.) Unfortunately, when less secure apps are not allowed, Outlook can't connect to Gmail servers, unless you also enable two-factor authorization then use an app password in Outlook. Other people said Google sent them an email recommending a security checkup or saying they were using less secure apps or and they took Google's advice to disable less secure apps. (The 'someone' turned out to be himself while using a hotspot.)
One of the steps was to turn off the allow less secure apps options, which he did.
He went through the steps and made the changes they suggested. While I was looking at his computer, he told me Google sent him an email because someone tried to connect to his account and the email included a list of steps to take to insure the account was secure. It was perplexing until one client wanted me to fix it for him. I started seeing a lot of Gmail users asking for help because Outlook stopped syncing.
