Hi, Valeria.
To reach San Gimignano from Siena you need the line 130/A (130/R for the return trip).
For Thursday or Friday you have to watch: “GIO” “VEN” “Feriale escluso il sabato” e “Tutti i giorni feriali”.
It’s possible that there are variations in the timetables from now until September.
For timetables and legend, next link and attach.