Europcar downtown I think moved its garage at the Firenze SMN station underground garage, although the office is still on Borgo Ognissanti.
Here are instructions from/to the SMN garage to Piazza Gaddi, outside the ZTL. Then from Piazza Gaddi use any navigator to lead you to San Gimignano. Just be aware of the speed camera on Ciale Etruria (if you take the A1 route) or the one on via Senese, if you take the via Senese route. Waze will warn you of both.
https://community.ricksteves.com/travel-forum/italy/car-rental-return-at-firenze-santa-maria-novella-station
If the Europcar personnel brings the car for you in front of their office, then below are the instructions out of Borgo Ognissanti. See my last comment in the thread below:
https://community.ricksteves.com/travel-forum/italy/car-rental-in-florence-airport-vs-train-station
Be aware that Europcar has 3 offices in Florence: the one downtown, described above, the one at Novoli, on via Forlanini, on the way to the airport and along the T2 light rail tramway to the airport, and lastly at the airport. Check the prices at their Novoli office as well. Since it’s not at the airport itself, there will be no airport extra fees from that office either, and that office is far outside the ZTL. If you pick up at that location, use any navigator to lead you to San Gimignano.