flake.nix

{
  description = "Launch Blade of Agony";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/867fe984b5299cf855be38ed9feed70a95a22eaf";
  };

  outputs = { self, nixpkgs, ... }: let
    pkgs = import nixpkgs { system = "x86_64-linux"; };

    makeGzWrapper = iwad: pkgs.writeShellScriptBin "gz-wrapper-${builtins.replaceStrings ["/" ":" "."] ["_" "_" "_"] iwad}" ''
      exec ${pkgs.gzdoom}/bin/gzdoom  -iwad ${iwad}
    '';

    gzdoomApp = iwad: {
      type = "app";
      program = "${makeGzWrapper iwad}/bin/gz-wrapper-${builtins.replaceStrings ["/" ":" "."] ["_" "_" "_"] iwad}";
    };
  in {
    defaultPackage.x86_64-linux = makeGzWrapper "boa.ipk3";

    apps.x86_64-linux = {
      boa = gzdoomApp "boa.ipk3";
    };
  };
}

run.sh

#!/bin/sh
unset LD_PRELOAD
export STEAM_RUNTIME=1
unset LD_LIBRARY_PATH
nix run .#boa

# unsets are to make it launchable with Steam so it can be used inside the gamescope big picture thing. 
# add this script as a non-steam game. 

Make sure you have the Blade of agony ipk3 file in the same directory.