app.rb

require 'sinatra'
require 'oauth2'
require 'json'

enable :sessions

# Scopes are space separated strings
SCOPES = [
    'https://mail.google.com/',
    'https://www.googleapis.com/auth/userinfo.email'
].join(' ')

unless G_API_CLIENT = ENV['G_API_CLIENT']
  raise "You must specify the G_API_CLIENT env variable"
end

unless G_API_SECRET = ENV['G_API_SECRET']
  raise "You must specify the G_API_SECRET env veriable"
end

def client
  client ||= OAuth2::Client.new(G_API_CLIENT, G_API_SECRET, {
                :site => 'https://accounts.google.com', 
                :authorize_url => "/o/oauth2/auth", 
                :token_url => "/o/oauth2/token"
              })
end

get '/' do
  erb :index
end

get "/auth" do
  redirect client.auth_code.authorize_url(:redirect_uri => redirect_uri,:scope => SCOPES,:access_type => "offline")
end

get '/oauth2callback' do
  access_token = client.auth_code.get_token(params[:code], :redirect_uri => redirect_uri)
  session[:access_token] = access_token.token
  @message = "Successfully authenticated with the server"
  @access_token = session[:access_token]
  
  # parsed is a handy method on an OAuth2::Response object that will 
  # intelligently try and parse the response.body
  @email = access_token.get('https://www.googleapis.com/userinfo/email?alt=json').parsed
  erb :success
end

def redirect_uri
  uri = URI.parse(request.url)
  uri.path = '/oauth2callback'
  uri.query = nil
  uri.to_s
end