gmail problem in my mail account

I have my gmail account in my Mail, so I can check from the Mail icon. Google has started sending me "We recently blocked a sign-in attempt to your Google account," when I open Mail, and wanting me to switch to the Gmail app, or change my security settings so that my account "is no longer protected by modern security standards". Although it claims the sign-in was blocked, it does download my emails, it's just annoying to keep getting those notifications from Google. Any ideas what this is all about? I can't imagine Mail is less secure than the Gmail app. Thanks!