GH is expressed either as degrees or as the equivalent amount of CaCO3 (calcium carbonate) - whether or not any calcium carbaonate is present. So, you can multiply degrees time 17.86 for mg/l. One degree is equal to 17.86ppm (or mg/l, both ppm and mg/l are numerically the same - 1ppm =1mg/l).
Note - this is US usage, other countries may use diferent concentrations or different scales.
There are no 'degrees' for nitrate. It can only be expressed as concentration. But some kits report nitrate - i.e., the whole ion, NO3-; others report nitrate-nitrogen, or only the amount of nitrogen in the sample, not the nitogen and oxygen combined in the nitrate. The conversion of nitrate-nitrogen to total nitrogen is by multiplying by 4.4, nitrate-nitrogen x 4.4 = total nitrogen.
HTH