There is no reason for women who are travelling to San Gimignano alone to be worried. Walking through the streets, both day and night, are safe. Nevertheless, if you need to feel more secure here is some guidance provided by http://www.lifeinitaly.com/travel/safety-tips.asp and telephone numbers should any need arise.
Police Telephone (Via del Castello; tel. 0577 940346)
Emergency Call (113)
Carabinieri Police Headquarters (tel. 0577 941312)