forward_email.rb

#!/usr/bin/env ruby
require 'optparse'
require 'rest_client'
require 'open-uri'

CLIENT_LOGIN_URI     = URI.parse('https://www.google.com/accounts/ClientLogin')
CLIENT_LOGIN_HEADERS = {'Content-type' => 'application/x-www-form-urlencoded'}
SERVICE              = 'mail'
ACCOUNT_TYPE         = 'GOOGLE'
SOURCE               = 'myaname-myprogram-0.0.1'
DOMAIN               = 'gmail.com'

def get_auth_token(email, password, logintoken=nil, logincaptcha=nil)
  params = {
      :accountType  => ACCOUNT_TYPE,
      :Email        => email,
      :Passwd       => password,
      :service      => SERVICE,
      :source       => SOURCE,
      :logintoken   => logintoken,
      :logincaptcha => logincaptcha
  }
  resp = RestClient.post(CLIENT_LOGIN_URI.to_s, params, CLIENT_LOGIN_HEADERS)
  lines = resp.body.split("\n")
  auth_line = lines.find {|l| l =~ /^Auth=.*/i}
  auth_line.sub(/^Auth=/i, '')
end

def setting_uri(domain, username, setting_id)
 URI.parse("https://apps-apis.google.com/a/feeds/emailsettings/2.0/#{domain}/#{username}/#{setting_id}")
end

def forwarding_uri(domain, username)
  setting_uri(domain, username, 'forwarding')
end

def forwarding_xml(forward_to, action="ARCHIVE")
  return <<-XML
<?xml version="1.0" encoding="utf-8"?>
<atom:entry xmlns:atom="http://www.w3.org/2005/Atom" xmlns:apps="http://schemas.google.com/apps/2006">
    <apps:property name="enable" value="true" />
    <apps:property name="forwardTo" value="#{forward_to}" />
    <apps:property name="action" value="#{action}" />
</atom:entry>
  XML
end

def settings_request_headers(auth_token)
  {
    'Content-type' => 'application/atom+xml',
    'Authorization' => "GoogleLogin auth=#{auth_token}"
  }
end

def strip_username(email)
  email.split('@').first
end

# ... optparse junk to set everything in @options ...

token = get_auth_token(@options[:email], @options[:password])
headers = settings_request_headers(token)
xml = forwarding_xml(@options[:forward_to])
# I presume the username has to be your email address without the @gmail.com
uri = forwarding_uri(DOMAIN, strip_username(@options[:email]))
# Raises RestClient::Unauthorized (401)
resp = RestClient.put(uri.to_s, xml, headers)