Volume is not mass, and neither of them is weight. A gram is strictly speaking a measure of mass, and we just consider it to be a unit of weight in casual terms because the only frame of reference the vast majority of us have has reasonably constant gravity so we conflate mass and weight. That you can sort of use grams to measure volume is literally only because the density of common stuff (especially water) is close enough for most purposes. It’s kinda like measuring a distance in units of time so long as the method of travel is known. I can say “an hour’s walk” and I’m not really measuring distance there but you know roughly how far I mean