There is no Gacutil included in the .net 4.0 standard installation. They have moved the GAC too, from %Windir%\assembly to %Windir%\Microsoft.NET\Assembly.
They havent' even bothered adding a "special view" for the folder in Windows explorer, as they have for the .net 1.0/2.0 GAC.
Gacutil is part of the Windows SDK, so if you want to use it on your developement machine, just install the Windows SDK for your current platform. Then you will find it somewhere like this (depending on your SDK version):
C:\Program Files\Microsoft SDKs\Windows\v7.0A\bin\NETFX 4.0 Tools
There is a discussion on the new GAC here: .NET 4.0 has a new GAC, why?
If you want to install something in GAC on a production machine, you need to do it the "proper" way (gacutil was never meant as a tool for installing stuff on production servers, only as a development tool), with a Windows Installer, or with other tools. You can e.g. do it with PowerShell and the System.EnterpriseServices dll.
On a general note, and coming from several years of experience, I would personally strongly recommend against using GAC at all. Your application will always work if you deploy the DLL with each application in its bin folder as well. Yes, you will get multiple copies of the DLL on your server if you have e.g. multiple web apps on one server, but it's definitely worth the flexibility of being able to upgrade one application without breaking the others (by introducing an incompatible version of the shared DLL in the GAC).
