NOTE: Achievements DO NOT work with this method! Tested it myself on the second account, and was reported by others. If you want to unlock the achievements, finish the game without this package. Maybe in the future, we get the achievements to work with it.

First and foremost, you need to download this:

Doom 64 CE

At the time of writing this guide, 3.4.0 was the latest version available.

In short, this package allows you to have all the benefits of the GZDoom source port.

Also you need Doom 64 re-release xD

Now comes the fun part.

Extract the archive you have just downloaded into your Doom 64 folder

SteamLibrary\steamapps\common\Doom 64

Once downloaded, either delete DOOM64_x64.exe or rename to DOOM64_x64.exe.bak


Now, you just rename gzdoom.exe to DOOM64_x64.exe (This is important if you want to use Steam overlay in game)

This is how your Doom 64 folder should look like


Now, you have installed everything need.

Before starting to play the game, one more step is needed to finish the installation. And this is important.
Reason for this is that the original Doom64.wad isn't compatible with GZDoom, and to make it compatible, click on doom64-install.bat.

This will automatically find Doom64.wad and patch it to make it compatible with GZDoom.

And that's basically it, you're done installing this. Open the game, select Doom64 CE from the menu, adjust the settings to your likings and have fun.

This is how it looks in game:



Oh, one more thing, since the Doom64.wad got patched, you may use it in Doom Builder to make your own levels for Doom 64 :D

Have fun!