The best bet for gravel is cyclocross (CX) tires. Gravel and light off-road is specifically what they're designed for. However 32–35mm is the typical size range.

If you want to stick with road tires, something with a reinforced sidewall, like Continental Ultra Gatorskins would be a good choice.

But, having ridden both road and CX tires in gravel, CX tires are by far the better choice, IMHO. Their shape and tread keep the gravel from squirting out from beneath the tires. In other words, they roll over the gravel, not through it.