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"[System.Reflection.Assembly]::Load("System.EnterpriseServices, Version=126.96.36.199, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")$publish = New-Object System.EnterpriseServices.Internal.Publish$publish.GacInstall("c:\mydlls\nameofmydll.dll")iisreset