The gacutil.exe should install the .dll to the right location depending on how it was compiled. You should be able to find the file here : %ProgramFiles%\Microsoft SDKs\Windows\v7.0A\bin\NETFX 4.0 Tools\
.NET 4 has it’s own Assembly folder (c:\windows\Microsoft.NET\assembly) which has a \GAC_32 and \GAC_64 directories within… This is where you will find your files once they’re installed.
Hope that helps…