Yes you are right, the dll needs to be strongly named in order to be registered in the GAC. However I just found out there is no need to do that if you are not performing a remote execution or using the dll from a remote location.
I also discovered a bug in Test Studio explaining the behavior you are experiencing. I created a custom dll and added it in the Project References. Then I went back to the code behind and tried to add a using statement, however my custom dll wasn't there. Then I restarted the project and it appeared as expected. See this video.
I filed a bug on this. You can track its progress and vote for the PITS Issue here: Public URL
I have also updated your Telerik points accordingly.
All the best,
the Telerik team