I'm fairly certain that the SITA bus system goes from Florence to San Gimigniano. I know that it goes to Siena and many other places in Tuscany. The website is www.sitabus.it but I've had difficulty in getting the site to come up in recent months. I'm unable to access it at the present time.
The SITA bus terminal is a long block directly south from the front of the Florence SMN train station. As you walk out, south is to your right. You need to cross the busy street and walk down a 1/2 block to see the terminal.
You can easily walk around San G. without a guide. The town is not really that big. The main street is nothing but shops and galleries. I believe that you can go up in some of the towers.