DOOMify is a suite of plug-ins for the GIMP to make converting textures for use in DOOM source ports with that DOOM 93 aesthetic easy.
Created a folder called "resized" (all lowercase) in the folder holding your full-sized textures. This is necessary for the resizing functions to export scaled items into a folder.
Simply copy (not cut) the folder of the version you are using into the GIMP user AppData folder. Use the file paths below as a guide.
Windows:
~/AppData/Roaming/GIMP/2.10/plug-ins