(only Episode 1). If you only have one episode, you are likely using the shareware file instead of the full Ultimate Doom compatible with The Ultimate Doom RetroPie-Docs/docs/Doom.md at master - GitHub
However, for the purist, or Woof! offer a "faithful" middle ground, preserving the original limitations while ensuring the game runs perfectly on modern hardware. the ultimate doom wad file
The Ultimate Doom WAD has had a profound impact on the gaming community. Its release marked a turning point in the evolution of Doom modding, as it provided a single, comprehensive file that contained all the necessary data for creating custom levels and modifications. (only Episode 1)
This is the easiest method.