The GNU Gatekeeper is free and very powerful. If you want to make your life a little easier, you should buy the web interface to have a graphical interface to configure and monitoring the GNU Gatekeeper. You'll be able to see ongoing calls, registrations and some statistics.
The web interface can be used from a desktop web browser as well as on mobile devices. It uses a responsive layout that adapts to your screen size.
The interface runs on a PHP enabled webserver on the GnuGk server (tested with Apache and IIS and PHP 5.x and PHP 7).
Please try the online demo (user name "demo", password "demo").
Price: 200 Euros