The usual way:

    C:\>gacutil.exe /u[f] <assembly name>

If it can't find gacutil.exe, check the %windir%\Microsoft.NET\Framework\v1.1.4322\ directory.

In this command, assembly name is the name of the assembly to uninstall from the GAC. The following example removes an assembly named hello.dll from the GAC.

    gacutil /u "hello,Version=1.0.0.0, Culture=neutral, PublicKeyToken=0123456789ABCDEF"

If you still can't uninstall it, here's a solution from Peter Huang [MSFT] that should work. The original post is at: http://bytes.com/topic/net/answers/49299-uninstall-assembly-gac#post366811

1. Run cmd in run dialog.
2. cd %systemroot%\assembly\gac\ClassLibrary1\1.0.1322.2 1592__22df95e3b227e638
(My test dll is ClassLibrary1.dll)
3. del ClassLibrary1.dll
4. del __AssemblyInfo__.ini
5. cd..
6. rd 1.0.1322.21592__22df95e3b227e638
7. cd..
8. rd ClassLibrary1

You may perform the steps above to see if the ClassLibray1 has been removed from the GAC.