Hey there. What exactly is the problem specifically? Are you getting an error message, or are the enemies just not being replaced? I just tested it in GZDoom 4.11.3 with no other mods loaded and can't find any problems.
Are you running GZDoom with any mods that replace monsters or change their behaviour? If so, it may be overwriting the mod's code, making it not work. Currently the mod is limited to working with stock Doom 2 enemies until I can find a way to replace them universally.