The aim of this open source project is offering a gs.apple.com proxy working in a server such as a Rasbperry PI.
Extract the project in the /srv folder of your server and review & integrate apache config in apache/ subfolder.
Add ''' server_ip gs.apple.com ''' in the /etc/hosts file of your clients.
Admin can be accessed at http://gs.apple.com/admin after completing setup. The source code is quite easy and commented. You may personalize it as you want.