hydrargyrum is a ☿ Mercury Protocol server — and through proxying also a Gemini Protocol server.
You need to have Go on your system to be able to build it yourself.
But, once you have Go on your system, you can build hydrargyrum with the following commands:
git clone https://github.com/reiver/hydrargyrum
cd hydrargyrum
go build
This will produce an executable program file named:
hydrargyrum
That executable program file is what you run to launch the ☿ Mercury Protocol server.