Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello Jason_MS56,
Welcome to Microsoft Community.
I am a Microsoft user just like you, I hope you are doing well.
Sorry for this inconvenience. However, if you are receiving that prompt, you cannot add that Gmail as an alias to your Microsoft account, there is no way around it as the system has detected your Gmail to be associated with a Microsoft account.
Source: https://support.microsoft.com/office/459b1989-356d-40fa-a689-8f285b13f1f2
Please let me know if you need more assistance.
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Warm Regards