The GameTracker Murmur plugin is required for adding servers to GameTracker. Instructions for setting it up can be found in the download package at http://www.gametracker.com/downloads/gtmurmurplugin.php

The plugin is a custom server side plugin which enables a "query port" for mumble servers.

is there a video on how to add it to a server?

GameTracker does not have a video tutorial, but there are instructions included in the readme.txt file.