To install WinPE in Ghost Solution Suite (GSS) 3.0, you first will need to install all prerequisites of the Windows ADK 8.1:
"To install the Windows ADK, your computer must be running one of the following operating systems:
• Windows 8.1
• Windows 8
• Windows 7
• Windows Server 2012 R2
• Windows Server 2012
• Windows Server 2008 R2
• Windows Vista
• Windows Server 2008
The Microsoft .NET Framework 4.5 is required and Windows ADK installs it automatically."
( http://www.microsoft.com/en-US/download/details.aspx?id=39982 )
NOTE: The last line indicated that .NET Framework 4.5 will be installed automatically if you don't have it, this requires a reboot. If you do not have .NET 4.5 already installed before attempting to add WinPE 5.1 to GSS 3.0, the reboot will occur while adding it which will result in .NET 4.5 being installed, but the rest of the process will cease and you will have to start over.
Once you have those prerequisites, download the adksetup.exe for Windows ADK 8.1: https://download.microsoft.com/download/6/A/E/6AEA92B0-A412-4622-983E-5B305D2EBE56/adk/adksetup.exe
Once the adksetup.exe is downloaded, open the Boot Disk Creator (for the Ghost Standalone Tools you will find it in the Start Menu, for the GSS Console you can use the link from within the console) and click Tools > Install pre-boot operating systems. Select the WinPE 5.X you want installed (x86 or x64, both can be installed but one at a time). Make sure the radio button by ADK Setup is selected, then click Browse and select the adksetup.exe that you downloaded in the previous step. After you click Next, be patient as it takes some time for adksetup.exe to download the rest of the Windows ADK and install it (this can be downloaded beforehand by running adksetup.exe yourself and selecting the option to download but not install. Alternatively you can install Windows ADK 8.1 yourself, the Deployment Tools and Windows Preinstallation Environment (Windows PE) are the only components required for this, then instead of selecting ADK Setup select Pre-installed ADK and point to the Windows Kits folder in Program Files (x86)).
Once this has completed, if desired repeat the process (using the same adksetup.exe, or selecting pre-installed ADK and pointing to Program Files (x86)\Windows Kits) to install the other WinPE (x86 or x64).