In the section called "On Google Compute Engine", this page says that authorization is handled for me with no configuration. But the example directly below shows using an explicit JSON key file:

var config = {
  projectId: 'grape-spaceship-123',
  keyFilename: '/path/to/keyfile.json'
};

var gcloud = require('gcloud')(config);

In fact, this is identical to the example used in the "Elsewhere" section.