Converting Windows Server 2012 Core to GUI is possible with Server Manager and PowerShell. But in Windows Server 2016 you are not able to convert Server 2016 Core to GUI. There is no more option for server 2016 convert from GUI to core or core to GUI.
Read the below Windows server released note for converting server core to GUI and vice versa.
Note for Windows Server 2016 Installation Options
After the release of Windows Server 2016 Technical Preview 2 in May, we heard from many of you about a change we made to the Server installation options. In Technical Preview 2, we made Minimal Server Interface the default and renamed the Server installation option to “Server with local admin tools”. The feedback was loud and clear that there are customer segments, most notably small and medium businesses, which require both the full graphical shell on their servers, and also a consistent experience with their Windows client desktops.
Based on your comments, we made the following change in Windows Server 2016 Technical Preview 3. The Server installation option is now “Server with Desktop Experience” and has the shell and Desktop Experience installed by default. This provides a more client-like experience on Windows Server for those that want a full local graphical experience on the server as well as for Remote Desktop Services Host.
As part of this work, some other changes have been made to the installation options. Because of the structural changes required to deliver the Desktop Experience on Server, it will not be possible to convert from Server with Desktop Experience to Server Core. It will also not be possible to convert Server Core up to Server with Desktop Experience.
So, the best option to have a GUI or Server core, you just chose the right option when you attempt to install Windows Server. On a local network server, you can easily install Windows server. But on online Windows dedicated server be careful to choose the right features for Windows Server installation.
Convert Server 2016 Core to GUI
Unlike some previous releases of Windows Server, you cannot convert between Server Core and Server with Desktop Experience after installation. If you install Server Core and later decide to use Server with Desktop Experience, you should do a fresh installation (and vice versa). But for Windows server 2012 and Windows server 2012 R2, read this article: Convert Server Core to Full Gui and Vice Versa.