There's one about 20-minutes from where I work that's real popular with the locals.
Mix Canyon
Vacaville ? The Diabolical Duo ? Mix Canyon & Gates Canyon (Super Steep) Hill Climbing
It's not all gravel but some of the gravel sections (not captured in those photos at the link above) are real brutal. A co-worker, 2-3 years ago chose it as his hill for one of his many Everest'ing challenges.
I've done the climb once, last fall and I did a lot of hike-a-bike. But my perceived fitness absolutely did not align with required fitness.