There are a number of articles you can find on the web which tell you how to do this. Unfortunately they all assume you started with the full fat server and subsequently removed the GUI or are otherwise incomplete in some way. I hope this will provide a step-by-step guide to adding the GUI to Server 2012 which started life as Core.
1. Find your copy of Windows 2012 or at least the install.wim.
2. Create a folder to mount a wim file in, eg c:\source
3. Run dism.exe /mount-wim /wimfile:d:\sources\install.wim /index:2 /mountdir:c:\source /readonly (the index in this case is the standard full server image, 1 is core).
4. Sit back and wait.
5. Run dism.exe /online /enable-feature /featurename:ServerCore-FullServer /featurename:Server-Gui-Shell /featurename:Server-Gui-Mgmt /source:c:\source\windows\winsxs
6. Sit back and wait even longer than before.
7. Reboot when prompted
8. Enjoy the GUI
