I hope your not going to add BOTH Sodium Bicarb and CO2, as they would cancel each others effects. (I use coral, calcium-carb, still buffers around 8.2)
NaCO3 + H20 --> Co2 + OH- (Thus raise pH), CO2 + h20 + Na+ (present in water) --> NaCo3 + H+ (acidic proton; lower pH).
GH is water hardiness? I think measure of Mg and Ca?
What do you want your pH to be? Need to find a buffer that will be around your pH.