'Calculus' and 'calculi' are common and _correct_ medical terms for stone/stones. There is no reason to use a different term. In this context 'calculi' obviously refers to stones in the gall bladder - not in the kidney - but 'gallstones' is more a layperson's term. 'Calculi' simply means 'stones' in medical writing. Any person with medical training, certainly ANY physician, would have no problem understanding 'calculi'. And physicians wouldn't use 'gallstones' in their reports; they would write 'calculi.'

By the way, our modern mathematical terms 'calculus' and 'calculate' come from the same word 'calculus' which in Latin referred to a pebble (=small stone) used for counting.