Surface-to-Evil Shotgun

Dune II has some great MIDI tracks that you could use for demonstration.

(Warning: I am not a lawyer. I've done a good amount of research on this, but this is not legal advice and I have not consulted with a lawyer about this.) With these specific circumstances, you actually could. You'd need a non-GPL "integration broker" that any game engine could communicate with via IPC. The integration broker would be in charge of redeeming achievements and performing match-making, among other things. Such integration broker would additionally support non-Steam frameworks for such things, too, such as RetroAchievements or Discord integration. I'm already planning on making such a program, but I'm not planning on implementing Steamworks support due to the fact that (to my knowledge) Valve doesn't make the Steamworks library publicly available without signing an NDA. An important note for your situation, though: GZDoom, last I checked, does not support players joining while the application is already running, which means you can't use a modern-style in-game party system.