Latest gacutil is part of Microsoft Windows SDK for Windows 7 and .NET Framework 4. Remember you can’t use previews version of gacutil (1.1 or 2.0/3.5) to register dll built in framework 4.0.
If you don’t want to install whole SDK you need to copy below files to server, only. Of course you need to install SDK on local machine to be able copy below files to server.
1. Files location: C:Program Files (x86)Microsoft SDKsWindowsv7.0ABinNETFX 4.0 Tools
List of files required to gacutil work properly:

• gacutil.exe
• gacutil.exe.config
• 1033gacutlrc.dll (can be placed in the same dir as gacutil.exe on the server)

Another interesting thing about assembly in 4.0 framework is location of your assembly. Location changed from C:Windowsassembly to C:WindowsMicrosoft.NETassemblyGAC_MSIL

Remember to unregister assembly you need to use commend:
gacutil /u assemblyname (you can’t use assemplyname.dll)

Additional information:
Understanding The CLR Binder
gacutil MSDN