This are few step on how you can convert a Windows Server 2012 Server Core Installation to a Full Installation (GUI).
The step id very straight forward.
What you need is only Windows Server 2012 installer & of course the Server 2012 Core itself..
For this demo, I'm using Windows Server 2012 Server Core that I install in Hyper-V Manager.
1 - Mount your Windows Server 2012 installer (.iso) to your Server Core in Hyper-V Manager
2 - on the cmd interface, create empty folder call mount by typing this command : mkdir c:\mount
3 - next, mount your Windows Server 2012 installer by typing this command and press Enter : dism.exe /mount-image /ImageFile:d:\sources\install.wim /Index:4 /Mountdir:c:\mount /readonly
4 - if everything ok, you will see Deployment Image Servicing and Management tool will appear and this dism.exe command now will mounting your install.wim file from your Windows Server 2012 installer.
5 - Once the mounting process completed, you need to start powershell function by typing PowerShell.exe
6 - In powershell interface, you need to load the ServerManager module by typing Import-Module ServerManager
7 - Next, you can now start install the Windows Server 2012 GUI components by typing this command : Install-WindowsFeature -IncludeAllSubfeature User-Interfaces-Infra -Source:c:\mount\windows
8 - after few minutes & process completed, you can now restart the Server by typing this command : shutdown /r /t 00
9 - your Windows Server 2012 should be restarted by now..
10 - Lastly, you can now have your nice Windows Server 2012 GUI and you can log in as usual by pressing CTRL + Alt + DEL.