The first thing you will need to do is find out what port Tixati is using.
In Tixati, go to: Settings -> Network -> Incoming Ports
Take note of the port number you see here. Next, open gufw and add allow rules for that port number for UDP and TCP.
You'll also likely have to port forward. Here's a general guide on how to do that: https://www.tixati.com/optimize/router-port-forward.html