Welcome to the Gulf Stream Owners RV Forum!
What model Gulf Stream do you have? You can list the information in you signature and it will be on every post like many of us have done.
I knew that Gulf Stream does drain the bathroom sink into the black tank when possible to provide the needed liquid for the tank, ours does.
But I didn't know about the kitchen sink. I wouldn't care for that either. I know for a fact that our kitchen sink does not drain into the black tank.
Ron