Gmail Account Blocked from Adding to Apple Mail (Other Gmail Accounts Work)

Hi all, I’m having trouble adding one Gmail account to Apple Mail — even though my other Gmail accounts are working fine in the app. This issue started after I updated passwords across accounts in response to the global security breaches reported in mid-June.

Here’s the situation:

• macOS Sequoia 15.5
• Apple Mail version 16
• Gmail account is personal (not Workspace)
• 2-Step Verification is deactivated
• Security Key was created using Apple Passwords
• Google continues to show a “Sign-in attempt was blocked” message when I try to add the account in Mail.
• I've confirmed the correct credentials - it's not the issue.

Any suggestions on how to get this one Gmail account working in Apple Mail again?

Thanks in advance!