Deleting a Google Cloud Platform domain mapping under Google App Engine

Google's current documentation on this topic doesn't match the actual user interface, at least for Google App Engine custom domain mappings.

Say you want to free up a domain that you've used on a different project on GCP or GAE.

When you try to use the domain, you get an error like:

error: [domain] is already mapped to a project

To solve this, use the command line (assuming you have the developer tools and so use gcloud regularly to deploy your apps) and do the following:

gcloud auth login

This will open a browser window for you to sign in with your Google account.

Set the project in the gcloud tool to the old project where the domain is currently mapped:

gcloud config set project PROJECT_ID

Replace PROJECT_ID with the actual Project ID of the old project.

List the domain mappings for the old project:

gcloud app domain-mappings list

Locate the domains you want to remove from the list, and then run the following command to delete the domain mapping:

gcloud app domain-mappings delete DOMAIN_NAME

... replacing DOMAIN_NAME with the actual domain you want to remove.

After a bit of waiting, you'll be able use the domain again in a custom mapping.