These steps are the same whether you're adding your first Gmail account or additional Gmail accounts to classic Outlook for Windows.
Select File > Add Account.
Enter your email address and select Connect.
Classic Outlook will launch the Gmail Sign in window. Confirm the email you're adding and select Next.
Note: If you receive a Something went wrong error message, select Retry and if prompted add your Gmail account info again.
Add your password and follow the prompts.
On the Google account permissions window, select Allow.
Once classic Outlook finishes adding your Gmail account, you can add another email account or select Done.
If you need instructions for adding an email account using advanced settings, go to Use advanced setup to add a POP or IMAP email account in classic Outlook for Windows.
Outlook won't accept my password
If classic Outlook won't accept your password and you know you're using the right password for your Gmail account, you might have an older version of Outlook that doesn't currently support this faster Gmail setup option. In that case, you'll need to use two-factor authentication and an app password to help verify that you're the person trying to access your Gmail account.
Gmail does allow you to add your account to classic Outlook without setting up two-factor authentication and using an app password, but you must turn on a setting that allows "less secure apps" to connect to your account. We do not recommend this setting. We recommend turning on two-factor authentication and using an app password.
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hth
Marcin