Question

why is Antonio compared to 'his grandsire in alabaster'?

Answer 1

Gratiano advises antonio not to be melancholic in nature and to be merryalso he tells him not to sit idle and to engage himself in his interestsso he is compared to his grandsire cut in alabaster(an idle or old man).also the elizabethians thought that jaundice was caused by the melancholic(in this case) influence of mind over body so gratiano says that under melancholic  influence of mind over body antonio is likely to be infected by jaundice.

Answer 2

Antonio is unhappy due to some unknown reason.Gratiano is trying to cheer him up.So hes asking Antonio to brush off his sadness and not sit like a statue of a grandfather made of white marble.Here Gratiano is asking him to be energetic and not sit like a statue.