Mike,
GacUtil.exe shouldn't be necessary--Windows Installer will handle this for you in a much cleaner fashion. Simply include the file inside a component, and set the Component Destination to [GlobalAssemblyCache].
What happens next is that InstallShield does a scan of the assembly (via the default setting of '.Net Scan at Build') to populate the tables necessary for talking to Fusion.dll, and Windows Installer handles the rest. It's really pretty easy.