
Click on the Green Button that is labeled as "Code". Go to the GitHub Repository for Project Brutality 3.0 in the "Getting Started" Section. Delete dosbox.exe, then rename gzdoom.exe to dosbox.exe. Copy all files from the GZDoom zip folder to your Steam Doom /base folder. That's ok, just apply the following steps below in that folder - as if you are treating it like it's a /base folder) Open /base folder (if you are in the Master Levels of Doom folder you will not see a /base folder. Open the GZDoom 7zip file you downloaded.
Install Doom then locate the folder in explorer ( Steam/steamapps/common/(Doom).
Download the latest version of GZDoom (currently 4.5.0).
Download the latest version of Project Brutality (currently 3.0).