Master Servers are 98% of the time headless applications. Why? Performance mostly, console, commandline applications are just enough for that. They are not build using game engines. It's something that works better when created from scratch (or using library/framework (like Kryonet), which is recommended). For example you can create server using C#, have your game made in Java, C++ or Python (I would like to say GoDot too, but not sure if GD supports that kind of networking without source editing) and it's going to work together by sending and receiving packets.
Secondly, I'm not working with GoDot (lack of community and extensions), never used networking and all so I'm not able to provide examples. Only documentation based stuff and overall experience.
What you should do is study as much as possible about how networking in GD works, so follow this documentation about High level multiplayer. There are examples that will help you progress further. Have in mind that GoDot won't give you working examples, you will have to rip your skin off before you get to the point where it will work as expected.