ChirpStack Gateway Bridge makes use of MQTT for publishing events and receiving commands. Mosquitto is a popular open-source MQTT broker, but any MQTT broker implementing MQTT 3.1.1 should work. In case you install Mosquitto, make sure you install a recent version.
Debian / Ubuntu
In order to install Mosquitto, execute the following command:
sudo apt install mosquitto
Please refer to the Mosquitto download page for information about how to setup Mosquitto for your platform.