Hi, Willhelm_555
Good day!
Thank you for posting to Microsoft Community. We are happy to assist you.
According to your description, the issue of your concern that you failed to add your Outlook email into G-mail app.
For narrowing down this issue, please be kindly to have a check on OWA - Outlook Web App and see if you can login it successfully or not.
If it shows the same error on OWA, we will stick with you and give you further assistance.
If it works on OWA, the issue can be narrowed down to the G-mail's side. In this situation, we could not give you an appropriate suggestion here. Please be kindly to contact G-mail's support for further assistance.
We would like to assist you and looking forward to your new updates.
Sincerely,
Alex | Microsoft Community Moderator.