You will definitely have to use marine salt, notaquarium salt, in a brackish tank. Also, if your tank is cycled and cycled as fresh water, you can not increase the salinity more than 0.2 per week or even slower without killing your bacteria. Never just add it to the tank, you need to dissolve it in water before adding.
Most plants will not make it in a brackish tank... sorry... I just learned that from a question posted on this board previously... I was hoping to set up a brackish and planted tank myself, since a web page advertised plants for use in a brackish set up, but that was corrected by RTR on this board.
As to the final sg of your tank I am not sure, hopefully someone else can answer the target sg for BB gobies.
I hope this helps you a bit.