Setting Google OAuth2 as the only login method

I want to deploy an instance of Gitlab for my team. We use Google as our main identity provider, so I’d want to enable signing up to Gitlab using Google only. I have deployed Gitlab CE on a DO droplet using their standard image, and updated the software to the latest version available on the repository (10.4.1).

I followed the guides for enabling OmniAuth and Google’s OAuth2. I have used the following settings in my /etc/gitlab/gitlab.rb:

gitlab_rails['omniauth_enabled'] = true
gitlab_rails['omniauth_allow_single_sign_on'] = ['google_oauth2']
gitlab_rails['omniauth_allow_sign_in_with_provider'] = 'google_oauth2'
gitlab_rails['omniauth_block_auto_created_users'] = false
gitlab_rails['omniauth_auto_link_ldap_user'] = true
gitlab_rails['omniauth_providers'] = [
  {
    "name" => "google_oauth2",
    "app_id" => APP_ID,
    "app_secret" => APP_SECRET,
    "args" => { "access_type" => "offline", "approval_prompt" => "" }
  }
]
gitlab_rails['sync_profile_from_provider'] = ['google_oauth2']
gitlab_rails['sync_profile_attributes'] = ['name', 'email', 'location']

However, attempting to sign-in with Google produces a 500 error. gitlab-ctl tail shows the following error:

==> /var/log/gitlab/gitlab-rails/production.log <==

Settingslogic::MissingSetting (Missing setting 'servers' in 'ldap' section in /opt/gitlab/embedded/service/gitlab-rails/config/gitlab.yml):
  lib/gitlab/ldap/config.rb:18:in `servers'
  lib/gitlab/ldap/config.rb:28:in `providers'
  lib/gitlab/o_auth/user.rb:111:in `ldap_person'
  lib/gitlab/o_auth/user.rb:104:in `creating_linked_ldap_user?'
  lib/gitlab/o_auth/user.rb:201:in `update_profile'
  lib/gitlab/o_auth/user.rb:13:in `initialize'
  app/controllers/omniauth_callbacks_controller.rb:107:in `new'
  app/controllers/omniauth_callbacks_controller.rb:107:in `handle_omniauth'
  app/controllers/omniauth_callbacks_controller.rb:9:in `block (2 levels) in <class:OmniauthCallbacksController>'
  lib/gitlab/i18n.rb:47:in `with_locale'
  lib/gitlab/i18n.rb:53:in `with_user_locale'
  app/controllers/application_controller.rb:324:in `set_locale'
  lib/gitlab/middleware/multipart.rb:93:in `call'
  lib/gitlab/request_profiler/middleware.rb:14:in `call'
  lib/gitlab/middleware/go.rb:18:in `call'
  lib/gitlab/etag_caching/middleware.rb:11:in `call'
  lib/gitlab/middleware/read_only.rb:31:in `call'
  lib/gitlab/request_context.rb:18:in `call'
  lib/gitlab/metrics/requests_rack_middleware.rb:27:in `call'

Where do I go from there?