What is the average concentration in gL and relative standard deviation RSD of chlorine in seawater if one cup of sea salt is used per gallon of water?

I suspect by gL you mean g/L. Borrowing from another Q/A here:
http://wiki.answers.com/Q/How_many_grams_of_salt_are_in_one_cup We'll assume 1 cup sea salt is 400 grams. 1 gallon = 3.7854L, so 1cup salt in 1 gal water = 400/3.7854 = 106 g/L You're asking for RSD in a way that doesn't make sense - maybe rephrase it or describe the information you're looking for.