What am I missing?

devise.rb

# (google_oauth2) Authentication failure! invalid_credentials: OAuth2::Error

config.omniauth :google_oauth2, 'key', 'secret',
    {
      :scope => ['https://www.googleapis.com/auth/userinfo.email', 'https://www.googleapis.com/auth/gmail.labels', 'https://www.googleapis.com/auth/userinfo.profile'],
      :prompt => "select_account",
      :image_aspect_ratio => "square",
      :image_size => 50,
      :include_granted_scopes => true,
      :access_type => 'offline'
    }

error.txt

(google_oauth2) Callback phase initiated.
(google_oauth2) Authentication failure! invalid_credentials: OAuth2::Error,

Gemfile.lock

oauth2 (1.1.0)
      faraday (>= 0.8, < 0.10)
      jwt (~> 1.0, < 1.5.2)
      multi_json (~> 1.3)
      multi_xml (~> 0.5)
      rack (>= 1.2, < 3)
    omniauth (1.3.1)
      hashie (>= 1.2, < 4)
      rack (>= 1.0, < 3)
    omniauth-google-oauth2 (0.3.1)
      jwt (~> 1.0)
      multi_json (~> 1.3)
      omniauth (>= 1.1.1)
      omniauth-oauth2 (>= 1.3.1)
    omniauth-oauth2 (1.4.0)
      oauth2 (~> 1.0)
      omniauth (~> 1.2)