Configuring preferences and options in Outlook.com
Thanks for getting back to us and for the information provided.
Since it was confirmed that you’re using an M365 account, then it’s possible that the Gmail account is added as an additional or separate mailbox in your Outlook account. Can you try navigating to OutlookSettings > Premium > Features > Additional mailboxes. Then, check if you’ll see the Gmail account from there and delete it afterward.
Should you have other queries that we can attend to, don’t hesitate to let us know.