Hello Joshua,
The error message you provided indicates that the domain is currently in use by some other project. You can
add custom domains using the Cloud Console, the Cloud SDK or through the App Engine APIs. In your particular case, the best way to re-add the domain to this particular project is to use the App Engine APIs since it gives you the additional option to override any existing mappings for the domain in question (assuming, of course, that the domain ownership has already been verified via the
Webmaster Central portal).
Here’s a brief set of instructions on how to use the API:
1. Visit the
API Explorer and authorize OAuth requests to be performed for your account
2. Enter your App ID under appsId, set overrideStrategy to OVERRIDE, and provide the domain in the body request as an “id” property.
You can enable SSL support afterwards in the Cloud Console by going to App Engine > Settings > Custom Domains, selecting the domain in question and clicking on “Enable managed security”.
As for the G Suite mapping, in order to remove a primary domain in the Admin Console, a second one has to be added and verified first. Then you can make the newly added domain your secondary, and later remove the original domain. This operation can cause issues with some G Suite services though, so I would suggest going for the API and Cloud Shell routes first.