Verify what account you're signed into Outlook with
These steps will show you how to verify which email account you're currently using in the Outlook desktop application:
Note: this does not apply to web mail, only to the Outlook app on your computer
1. Open the Outlook app on your computer
2. Click the gear icon in the top right corner
3. Check under Account Details to see which account you're currently using
If you're Gmail account is listed, you will need to remove it before you can add your Microsoft account. Follow the articles below to complete those steps.
How to remove Gmail from Outlook
How to add your Microsoft email to Outlook