The default image name in the code has a bug. It is being set to packer-{{timestamp}}, the {{timestamp}} part needs to be interpolated. Without the interpolation the GCE builder fails with
the following error

==> googlecompute: Creating image...
==> googlecompute: Error waiting for image: googleapi: Error 400: Invalid value for field 'resource.name': 'packer-{{timestamp}}'. Must be a match of regex '(?:[a-z](?:[-a-z0-9]{0,61}[a-z0-9])?)', invalid