You should be able to download the "Google Cloud Platform credentials JSON file" ( google.json in Welcome to certbot-dns-google’s documentation! — certbot-dns-google 0 documentation )
These names can be arbitrary; it is only important that you remember them. Under "Key type," we recommend that you leave this value as "JSON". Click "Create", and the GCP Console will generate a JSON key (as a .json text file), prompt you to download the file to your computer, and display a Service account created dialog box.