config.ru

begin
  # Require preresolved locked gems
  require ::File.expand_path('.bundle/environment', __FILE__)
rescue LoadError
  # Fallback on resolving at runtime
  require 'rubygems'
  require 'bundler'
  Bundler.setup
end

require 'sinatra/base'
require 'gowalla'

class App < Sinatra::Base

  set :sessions, true

  get '/' do
    if session[:access_token]
      redirect '/auth/gowalla/test'
    else
      "Why not authenticate with <a href=\'/auth/gowalla\'>Gowalla</a>?"
    end
  end

  get '/auth/gowalla' do
    redirect(client.web_server.
             authorize_url(:redirect_uri => redirect_uri, :state => 1))
  end

  get '/auth/gowalla/callback' do
    session[:access_token] = client.web_server.get_access_token(params[:code], :redirect_uri => redirect_uri).token

    if session[:access_token]
      redirect '/auth/gowalla/test'
    else
      "Error retrieving access token."
    end
  end

  get '/auth/gowalla/test' do
    unless client.needs_access?
      u = client.user('pengwynn')
      "Where's Wynn? He last checked in at #{u.last_checkins.first.spot.name}."
    else
      redirect '/auth/gowalla'
    end
  end

  protected

  def client    
    Gowalla::Client.new (
      :api_key     => "your_api_key",
      :api_secret  => "your_api_secret",
      :access_token => session[:access_token]
    )
  end

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

end

run App

Gemfile

source :rubygems

gem 'sinatra', '1.0'
gem 'gowalla', '~> 0.2.0'

group :development do
  gem 'shotgun'
end