I’ve done the last few Eroica rides and I am planning to ride this year.
Riding on Saturday, you will need to try to arrive in Gaiole by 6am for the start of the 135KM. The challenge is that the main road into Gaiole, SP408 is the main route out for the the riders on the 209K, so traffic into town is heavily controlled.
A secondary route is from Radda from SP429, and take the Strade Bianche from Vertine. Before you reach Gaiole, you will find a large parking lot that is for the event.
Post ride, the event goes well into the evening, so traffic will not be bad leaving.
You will be able to pick up the rental bike the day before the event, as well as the race packet.
It is worth the effort - best of luck.