Russian Overkill Deathmatch stuff

First off, seeing as you'll be using GZDoom you'll only want to play against one other person. When you play GZDoom with more than two people, you play with Hitler. The other players desync in seconds and are forced to quit.

You'll both want the latest GZDoom SVN, found here: http://svn.drdteam.org/gzdoom/

You can work with command line if you know it better (and actually I find it more useful as you can set server settings before beginning the game), but this launcher makes things really easy for the client:
http://www.doomworld.com/vb/source-ports/50388-gzdoom-multiplayer-launcher/

You'll only be able to play one map in a session, so pick a good one. Size should definitely be larger than your average Doom 2 map, but also not as big as Doom2-4xbigger. For minimal chance of desync, make sure you're both using the GL renderer and you're both loading the wads in the exact same order.

Some tips to make it a more enjoyable experience:
-Don't use the Comrade altfire. It's just a bitch move for everyone involved.
-Don't use the Beef Supreme's turret function. The turrets can't be destroyed, never run out of ammo, and don't reward frags. They exist only to shit all over spawn points.
-Play with sv_respawnsuper false. Nobody likes respawning megaspheres.