Why can't I see all GAC comp in c# 2005 Express?

Question

I've downloaded a class library's source from the internet and built it in the Visual c# 2005 Express Ed succesfully. Afterthat I've Installed the dll to the Gac succesfully. I can see it in the directory "C:\WINDOWS\assembly". But I can't see either my component or some other components being in the GAC in the "Choose Toolbox Items dialog / .NET Framework components tab" of Visual c# Express 2005? What conditions the GAC assemblies have to meet to be seen in the "Choose Toolbox Items dialog" ? My dll may use Framework 1.0...can it be a problem? How can I determine which framework it use?