Why you don't add the doom 64 wad's too? Nice program btw
Doom 64 is quite different from standard Doom under the hood. That's why there was a separate project Doom 64 EX and now the official re-release that's available. It would require a ton of work to include into GZDoom, and it would only benefit this single game/wad... it would essentially need to be a whole different "mode" just for this one game.