Overview
You can access the folders associated with the GTGD S3 tutorial through the convenient menu system provided by the developer but I thought it might help to break down the file locations that come with this tutorial.
Location
In Steam right click on GTGD S3 icon, scroll down to select properties. The properties box has a local files tab, from there click on browse local files. This will take you to the location of the GTGD S3 files in Steam.
Demo Build
After opening the GTGD S3 Launcher_Data folder you will see a few more folders. The demo build folder contains the completed project compiled into a launchable game. This folder can be copied anywhere more convenient and run from the GTGD S3 Demo Build.exe within the Demo Build folder.
Scripts
Can’t figure out where you went wrong? The Scripts folder contains finished scripts you can use to check your work.
Within the Scripts folder is a complete Unity project on multiplayer in Unity 5. The Unity 5 Simple Multiplayer/Assets/Simple Multiplayer/My Scenes contains a project scene that can be opened in Unity.
Not to worry about messing the files up, with Steam you can verify files and restore the files back to original. However if you want to play with the files I suggest you copy the entire Unity 5 Simple Multiplayer folder to another location and open that folder in Unity.
Videos
If you would like to play the videos outside of Steam you can find them in this folder.