The only way to get at the GAC is by calling into the unmanaged fusion
API's.
Following is a complete sample that illustrates how you can perform a name
lookup.
Beware that the GAC uses fully qualified assembly names to store assembly
references, because you don't know whether an assembly resides in the GAC,
you also don't have a FQAN, this may result in false positives!

Willy.

// Note that this requires V2 of the framework.
using System;
using System.Runtime.InteropServices;
using System.Text;

namespace GacStuff
{
   internal class GacApi
   {
       [DllImport("fusion.dll")]
       internal static extern int CreateAssemblyCache(
               out IAssemblyCache ppAsmCache,
               int reserved);

   }
   // GAC Interfaces - IAssemblyCache. Non used vtable entries declared as
dummy.
   [ComImport, InterfaceType(ComInterfaceType.InterfaceIsIUnknown),
Guid("e707dcde-d1cd-11d2-bab9-00c04f8eceae")]
   internal interface IAssemblyCache
   {
       int Dummy1();
       [PreserveSig()]
       int QueryAssemblyInfo(
                           int flags,
                           [MarshalAs(UnmanagedType.LPWStr)]
                           String assemblyName,
                           ref ASSEMBLY_INFO  assemblyInfo);
       int Dummy2();
       int Dummy3();
       int Dummy4();
   }
    [StructLayout(LayoutKind.Sequential)]
   internal struct ASSEMBLY_INFO
   {
       public int      cbAssemblyInfo;
       public int      assemblyFlags;
       public long     assemblySizeInKB;
       [MarshalAs(UnmanagedType.LPWStr)]
       public String   currentAssemblyPath;
       public int      cchBuf;
   }

   class Program
{
 static void Main()
 {
  Console.WriteLine(QueryAssemblyInfo("System"));
 }
       // If assemblyName is not fully specified, a random matching will be
returned!!!!
       public static String QueryAssemblyInfo(String assemblyName)
       {
           ASSEMBLY_INFO  assembyInfo = new ASSEMBLY_INFO ();
           assembyInfo.cchBuf = 512;
           assembyInfo.currentAssemblyPath = new String('\0',
assembyInfo.cchBuf) ;
           IAssemblyCache assemblyCache = null;
           // Get IAssemblyCache pointer
           int hr = GacApi.CreateAssemblyCache(out assemblyCache, 0);
           if (hr >= 0)
               hr = assemblyCache.QueryAssemblyInfo(1, assemblyName, ref
assembyInfo);
           else
               Marshal.ThrowExceptionForHR(hr);
           return assembyInfo.currentAssemblyPath;
       }
}
}
//-------------------------------