According to the wizard, I should share my google calendar with the service account ID. I only have the option to share it with the email address that google assigned to the service account (the field is only fillable with an email address).
On step 7 of the wizard, I see this:
Following wizard to import my existing google calendar.
I'm not sure if there's something I need to do differently in my calendar settings, service account settings, or Drupal.
Downport
2.5
Documentation
The future of the module is using the google_client_api module, and this doc is specific to the old google_secrets module. The documentation needs update to the 3.0.x branch.