You need to delete them from the registry -
- open the registry editor
- go to Computer\HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\Profiles
- open your profile key - it's probably called Outlook.
- look for this key - 9375CFF0413111d3B88A00104B2A6676 - it will have a triangle next to it.
- expand each of the subkeys (000000*) and look for the gmail account - if you have 3, it should be the last 3.
- If you were finally able to add the account, the first 2 are the ones to delete. Or just delete it from account settings and remove the keys that are left.
You may also want to search the profile key for the value 001f6641
- if you open it, double click to open and see if the gmail account is listed. If so, delete it. (This works best if you don't have a working Gmail account in your profile - otherwise you might delete the key for it.)