Path of DLL installed to the GAC

If something gets put in the GAC, it actually gets copied into a spot under %WINDIR%\assembly, like

C:\WINDOWS\assembly\GAC_32\System.Data\2.0.0.0__b77a5c561934e089\System.Data.dll

I assume you're seeing something like that when you check the Location of the assembly in question when it's installed in the GAC. That's actually correct. (In .NET 1.1 there was a "Codebase" listed when you looked at a GAC assembly's properties, but that was only to show you where the original file was located when you ran gacutil - it didn't actually indicate what would be loaded.) You can read more about that here.

Long story short, you may not be able to do what you want to do. Instead of looking in relation to some assembly that's being loaded (Assembly.GetExecutingAssembly()), you might want to switch the behavior to look relative to the primary application assembly (Assembly.GetEntryAssembly()) or put the file in some well-known location, possibly based on an environment variable that gets set.

Do you have the option of embedding a resource to this DLL? That way, it doesn't really matter where the DLL is located on disk, because the XML file will follow it. You can then do something like this:

Stream s = Assembly.GetExecutingAssembly().GetManifestResourceStream("MyProject.MyXmlFile.xml");
XmlDocument d = new XmlDocument();
using (StreamReader r = new StreamReader(s))
{
    d.LoadXml(r.ReadToEnd());
}

After the assembly is shadow copied into the Global Assembly cache, i don't think there is any metadata to traceback the location of the source assemblies.

What are you trying to achieve by deploying in GAC? If its just for the sake of CLR for resolving purposes, then there is an alternate way that solves your problem.

Don't gac install the dll, rather add the following key in the registry, (this registry location is looked up by CLR when trying to resolve assemblies)

32 bit OS : HKEY_LOCAL_MACHINE\SOFTWARE\\Microsoft\.NETFramework\v4.0.30319\AssemblyFoldersEx\foo

64 bit OS : HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v4.0.30319\AssemblyFoldersEx\foo

For the foo key (Use your favourite name instead of foo), you will see a Key Name "Default". Double click it and set the value to wherever your assembly exists. (absolute path is preferred)

Now from Visual Studio, your client should be able to see your assemblies in the "Add Reference" Dialog and can use it.

Now coming to your actual problem,

Assembly.GetExecutingAssembly() will return the path of the location where the insatlled dll's are present. Find the XML file from there. :)

Note: In the registry key the 4.0.30319 is the version of the .NET Framework your application targets. Use whatever version your application targets instead.