I've done that Geysers route before, but in March. Our local club does a weekend ride there every year.

The temperature at the start can be brisk, but I warmed up quickly enough to lose the gilet for the rest of ride. Arm and knee warmers, summer gloves, lightweight gilet.

That one steep bit away from the Geysers is pretty short, nothing to be concerned about.

This is the gilet I bring when it's below 60 F: Voler Men's Vellum Rain Vest. Windproof, water resistant, stretchy, packs very small.

But if it's raining, I'd skip the ride.