Hi Brian Rang,
The “already exists” message usually means that your Gmail address is already linked to a different Microsoft account, even if you do not remember creating or using that account.
First, try signing in to Microsoft using the Gmail address. If it opens another Microsoft account, go to the Manage how you sign in to Microsoft page for that account. Make sure the account has another working sign-in address, set that address as primary if necessary, and then remove the Gmail address.
After the Gmail address has been removed from the other account, sign back in with your Comcast-based Microsoft account, add the Gmail address as an existing email alias, verify it, and select Make primary.
If signing in with Gmail does not locate the account, try the Microsoft account Sign-in Helper. Please note that two separate Microsoft accounts cannot be merged, so the Gmail address must be removed from the account currently using it before it can be added to the account you want to keep.
Can you currently sign in to a Microsoft account using that Gmail address, or does Microsoft say that no account exists? Thank you so much for your understanding.
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