[.net] where is gacutil.exe?

On Windows 2012 R2, you can't install Visual Studio or SDK. You can use powershell to register assemblies into GAC. It didn't need any special installation for me.

Set-location "C:\Temp"
[System.Reflection.Assembly]::Load("System.EnterpriseServices, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")
$publish = New-Object System.EnterpriseServices.Internal.Publish
$publish.GacInstall("C:\Temp\myGacLibrary.dll")

If you need to get the name and PublicKeyToken see this question.