Adding .NET DLL into GAC does not make it automatically appear in the
reference dialog in VS.

There is two way to solve it:

1. Directly set reference to the DLL file. This is recommended approach.
Although iin your new project the reference is set to the DLL file, when
your new app runs, .NET always search GAC first and if fond, load it from
there.

2. You have to modify Windows Registry. I could not remember the details
('cause I only use the first approach aforementioned). Search the net would
lead you to the detail of how to do it (again, I wouldn't bother doing it,
if I use DLL developed by myself).