If you install Windows Server 2012 with a core installation (without the GUI) and later find that you would like to install the GUI you may do so using the following steps.
1. Use the Windows PowerShell command Get-WindowsImage -ImagePath <path to wim>\install.wim in order to determine the index number for a server with a GUI image. If the path to the install.wim file is D:\sources\install.wim then the command would be Get-WindowsImage -ImagePath D:\sources\install.wim.
2. Use the Windows PowerShell command Install-WindowsFeature Server-Gui-Mgmt-Infra, Server-Gui-Shell –Restart –Source wim:<path to wim>\install.wim:<Index # from step 1>. For example, if you choose wim #4 (Windows Server 2012 SERVERDATACENTER) then the command would be Install-WindowsFeature Server-Gui-Mgmt-Infra, Server-Gui-Shell –Restart –Source wim:D:\sources\install.wim:4.
NOTE: This entry assumes that the Windows Server 2012 installation media is in the D: drive.