This article should help you out:
Gmail Not Working in Outlook 365? Here’s the Proven Fix (2026)
See the steps below to fix Gmail in Outlook classic during Setup:
Press Windows + S, type Credential Manager, and open it.
Click Windows Credentials.
Scroll to Generic Credentials.
Find entries starting with MicrosoftOffice16_Data:OAUTH2.
Expand each entry.
Hover over MicrosoftOffice16_Data:OAUTH2 to see the full name containing tp_google_imap_Oauth2 (these are Gmail credentials).
Click Remove for each MicrosoftOffice16_Data:OAUTH2 entry.
Remove both credentials for the affected Gmail account.
Optional: To identify the correct Gmail account before deleting:
Open Registry Editor.
Paste this path in the address bar:
Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity\Identities
Check the Identity subkeys.
The number at the beginning of the identity key matches the number in the MicrosoftOffice16_Data credential.
After removing the credentials:
Open Outlook.
Go to the affected profile.
Click Sign in via browser in the Outlook notification bar.
Sign in to your Gmail account again.
Remove access for Microsoft Apps & Services in Gmail (Fix Gmail in Outlook Sync)
Close Outlook.
*May not be needed*
Go to google.com and sign in to your Google account.
Click your profile picture in the top-right corner.
Select Manage your Google Account.
From the left menu, click Security.
Scroll to Your connections to third-party apps & services.
Select Microsoft apps & services.
Find Microsoft apps & services has some access to your Google Account.
Click See details.
Under Access you’ve given to Microsoft apps & services, click Remove all access. This should reset the Gmail in Outlook sync, and you can try again.
Select Confirm.