Signet Issue

CODE:
require 'signet/oauth_2/client'

  gapi_client = Signet::OAth2::Client.new(
    :authorization_uri => "https://accounts.google.com/o/oauth2/auth",
    :token_credential_uri => "https://accounts.google.com/o/oauth2/token",
    :client_id => Rails.application.secrets.gapi_client_id,
    :client_secret => Rails.application.secrets.gapi_client_secret,
    :redirect_uri => Rails.application.secrets.gapi_redirect_uri,
    :scope => "https://www.googleapis.com/auth/plus.login")

ERROR: uninitialized constant Signet::OAth2
GEMFILE:
...
#Google Plus API OAth2 Client\
#Github: https://github.com/google/signet
gem 'signet', '~> 0.5.1'
...