I thought GH only measured the cations. If you have 60 ppm of divalent cations and your GH booster is mostly sulfate salts, then you have 1:1 cations tlo anions. Then 120 ppm on TDS makes sense (60 ppm Ca2+/Mg2+, 60 ppm SO42-), as it is Total Dissolved Solids, not Positive Dissolved Solids.
However, I do not have a TDS meter myself. So do take whatever I say with a grain of salt.