Find the folder where your gzdoom.exe or equivalent is located.
A .pk3 file is a data archive (essentially a ZIP file) used by modern Doom source ports. You cannot run this file by itself. doom-complete.pk3
A: No. While the Unity port includes some similar QoL features, doom-complete.pk3 is strictly for Source Ports like GZDoom. It is not compatible with the official Bethesda remaster. However, WadSmoosh does import the "Ultra-Violence+" skill level originally found in the Unity port. Find the folder where your gzdoom