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").
Already included if you buy the GnuGk Appliance.
Price: 200 Euros (license for 1 server)