If you have no rental car, go towards Piazza Gramsci station and buy a ticket to San Gimignano. It is bus 130 (65min, 6 €) and it will leave at the main city gate (Porta San Giovanni). Find the hours at this schedule. Take into account that some buses stop at Poggibonsi, where you have to change to another bus to reach San Gimignano. They will indicate you if that is the case.
