controller that simulates a Braintree gateway. The "trade" stuff was specific to our app, so update for your situation

braintree_simulation_controller.rb

class BraintreeSimulationController < ApplicationController
  VALID_CC_NUMBERS = [ '4111111111111111', '5431111111111111', '6011601160116611', '341111111111111' ]
  
  def transact
    if params[:merchant_defined_field_1] == 'billing'
      current_user.update_attributes( :bill_to_first_name => params[:firstname],
                                      :bill_to_last_name => params[:lastname],
                                      :bill_to_address => params[:address1],
                                      :bill_to_address2 => params[:address2],
                                      :bill_to_city => params[:city],
                                      :bill_to_state_abbr => params[:state],
                                      :bill_to_zip => params[:zip],
                                      :bill_to_daytime_phone_area_code => params[:daytime_phone_area_code],
                                      :bill_to_daytime_phone_prefix => params[:daytime_phone_prefix],
                                      :bill_to_daytime_phone_line_number => params[:daytime_phone_line_number],
                                      :bill_to_daytime_phone_extension => params[:daytime_phone_extension])
    end

    case
      when !VALID_CC_NUMBERS.include?(params[:ccnumber]) then invalid
      when params[:amount].to_f < 1 then declined
      else success
    end
  end

  private

    def braintree_response_with(response_params)
      trade = Trade.find(params[:orderid])
      redirect_to make_trade_trade_url(trade, default_params.merge(response_params))
    end
    
    def default_params
      from_request = params.slice('type', 'amount', 'time', 'hash', 'orderid')

      from_request.merge 'avsresponse' => '', 
                         'username' => PAYMENT_CONFIG[:key_id], 
                         'cvvresponse' => ''
    end

    def invalid
      braintree_response_with 'response' => '3', 
                              'responsetext' => 'Invalid Credit Card Number REFID:304746995', 
                              'authcode' => '', 
                              'transactionid' => '0', 
                              'response_code' => '300'
    end

    def declined
      braintree_response_with 'response' => '2', 
                              'responsetext' => 'DECLINE', 
                              'authcode' => '', 
                              'transactionid' => '1083050711',
                              'response_code' => '200'
    end

    def success
      braintree_response_with 'response' => '1', 
                              'responsetext' => 'SUCCESS', 
                              'authcode' => '123456', 
                              'transactionid' => '1081825527', 
                              'response_code' => '100'
    end

end

fakeweb_helpers.rb

module FakewebHelpers  
  # Make sure nothing gets out (IMPORTANT)  
  FakeWeb.allow_net_connect = false  
  
  # Turns a fixture file name into a full path  
  def fixture_file(filename)  
    return '' if filename == ''  
    File.expand_path(RAILS_ROOT + '/spec/fixtures/' + filename)  
  end  
  
  # Convenience methods for stubbing URLs to fixtures  
  def stub_get(url, filename)  
    FakeWeb.register_uri(:get, url, :body => fixture_file(filename))  
  end  
  
  def stub_post(url, filename)  
    FakeWeb.register_uri(:post, url, :body => fixture_file(filename))  
  end  
  
  def stub_any(url, filename)  
    FakeWeb.register_uri(:any, url, :body => fixture_file(filename))  
  end 
  
  def stub_braintree_transaction_query(filename)
    url = %r|https://secure.braintreepaymentgateway.com/api/query.php|
    stub_get url, "braintree/#{filename}"
  end 
end  
  

routes.rb

  map.braintree_simulation '/braintree_simulation/:action', :controller => 'braintree_simulation'