Usage: Gacutil <command> [ <options> ]
Commands:

 /u <assembly_name> [ /r <...> ]
   Uninstalls an assembly. <assembly_name> is the name of the assembly
   (partial or fully qualified) to remove from the Global Assembly Cache.
   If a partial name is specified all matching assemblies will be
uninstalled.
   Example:
     /u myDll,Version=1.1.0.0,Culture=en,PublicKeyToken=874e23ab874e23ab
     /r FILEPATH c:\projects\myapp.exe "My App"

 /uf <assembly_name>
   Forces uninstall of an assembly by removing all traced references.
   <assembly_name> is the full name of the assembly to remove. Assembly will
   be removed unless referenced by Windows Installer.
   ! Warning: use the /uf command with care as applications may fail to run !
   Example: /uf
myDll,Version=1.1.0.0,Culture=en,PublicKeyToken=874e23ab874e23a
b

 /ul <assembly_list_file> [ /r <...> ]
   Uninstalls one or more assemblies from the global assembly cache.
   <assembly_list_file> is the path to a text file that contains a list of
   assembly names. Individual names in the text file must be
   separated by CR/LF.
   Example: /ul myAssemblyList.txt /r FILEPATH c:\projects\myapp.exe "My App"
     myAssemblyList.txt content:
     myDll,Version=1.1.0.0,Culture=en,PublicKeyToken=874e23ab874e23ab
     myDll2,Version=1.1.0.0,Culture=en,PublicKeyToken=874e23ab874e23ab

 /ungen <assembly_name>
   Uninstalls a native image installed via the NGEN utility. <assembly_name>
   is the name of the assembly for which native images are to be uninstalled.
   The assembly itself will remain in the global assembly cache.
   Example: /ungen myDll

Hope this helps.
Regards
Sam