On windows 2012 server drag&drop installation of dlls to GAC is not working anymore. When you try to use gacutil.exe you do not have it by default anymore as well:)

One way is installing SDK or Visual Studio and using gacutil from there.

The other and easiest way to copy a dll to gac on windows 2012 is using powershell.

In order to do this open a powershell console as an administrator. Copy your dll's to a folder in the machine lets assume the folder is c:\mydlls and name of the dll is nameofmydll.dll

Run the following commands and powershell will copy your dlls to GAC for you :)

Set-location "c:\mydlls"