google_oauth.rb

require 'oauth'
require 'oauth/consumer'
require 'oauth/signature/rsa/sha1'

APP_URL        = 'http://example.com'
OAUTH_KEY      = 'key'
OAUTH_SECRET   = 'secret'
OAUTH_APP_NAME = 'MyAppName'

get '/auth' do
  consumer = get_consumer
  next_url = "#{APP_URL}/auth/create"
  request_token = consumer.get_request_token({:oauth_callback => next_url}, {:scope => "http://docs.google.com/feeds/ http://spreadsheets.google.com/feeds/"})
  session[:oauth_secret] = request_token.secret
  redirect request_token.authorize_url
end

get '/auth/create' do
  request_token = OAuth::RequestToken.new(get_consumer, params[:oauth_token], session[:oauth_secret])
  access_token = request_token.get_access_token(:oauth_verifier => params[:oauth_verifier])
  session[:token_string] = access_token.token
  session[:token_secret] = access_token.secret
  redirect '/' # done with auth, now redirect somewhere in your app
end

get '/logout' do
  session[:token_string] = session[:token_secret] = nil
  redirect '/'
end

def get_consumer
  consumer = OAuth::Consumer.new(
    OAUTH_KEY, OAUTH_SECRET,
    {
      :site               => "https://www.google.com",
      :request_token_path => "/accounts/OAuthGetRequestToken",
      :access_token_path  => "/accounts/OAuthGetAccessToken",
      :authorize_path     => "/accounts/OAuthAuthorizeToken",
      :signature_method   => "HMAC-SHA1"
    }
  )
end

def get_access_token
  session[:token_string] && session[:token_secret] && OAuth::AccessToken.new(get_consumer, session[:token_string], session[:token_secret])
end