This is a GDB server for Z80 processor. It's made as a plugin to ZXMAK2 ZX Spectrum emulator. More info about emul you can find at (in Russian, use translator):
http://zx.pk.ru/showthread.php?t=16830

You should use gdb-z80 (https://github.com/legumbre/gdb-z80) as a client.
Connection can be performed by gdb command
  
  target remote localhost:2000

If you are interested in standalone Z80 GDB server please take a look at 'stand-alone' tag (it uses few classes from ZXMAK too)
--
Alexander Tsidaev