Managing personal Outlook.com account settings, security, and privacy
Dear starrxu
I'm sorry for the slightly late reply.
I've run various tests and am currently unable to use GMAIL's built-in email sending and receiving on my behalf.
My side is consistent with your reported error.
I tested various states of the account, such as turning on two-step verification, using a separate password, or simply using a password.
None of them can pass the authentication and I believe this is Gmail temporarily turning off the ability to send and receive emails on behalf of the account.
If you have a Microsoft 365 subscription, it is even more recommended that you use Classic Outlook directly to configure and manage all your mailboxes.
We're sorry to see that you've marked it as unhelpful, but we're eager to resolve your issue. You can tell us the details of your needs and we may have other ways to help you realize them.
In particular, the issue you have raised is a potentially deprecated feature in Gmail.
Best Wish
Shawn.Z-MSFT | Microsoft Community Support Specialist