Your English is hard to decode but it sure sounds like you are on the wrong track with this. The GAC is a deployment detail. You'll have no trouble putting an assembly in the GAC with your Setup project.
That assembly does not belong in the GAC on your dev machine. To add the reference, simply use the Add Reference dialog and use either the Project or Browse tab.
If your assembly is intended to be used by another programmer so that she can use your product in her own project then you need to deploy your assembly twice. It needs to go into the GAC and a copy of the assembly needs to go in a dedicated folder so your client can add the reference to it in her own project. The c:\program files\reference asssemblies directory is a good place to put it.
Putting it in the GAC for her is optional btw, it isn't that commonly done. I think most programmers would prefer that you don't do this, unless you work for a Big Company that needs to automatically distribute security updates. Putting it in the GAC just makes it harder for your client to write her own Setup project since she has to run yours first.