For loose gravel, I believe what you want is big, wide tires that you run at a relatively low pressure. I don't think knobs will help much.
The wider the tire, the more it will press the loose gravel down instead of pressing through it. If the gravel is pressed down it will tend to stay more in place. Also, if some of it shifts, a wider tire will be more likely to have grip on something that isn't shifting.
I don't think knobs would help unless the gravel's fine enough that the knobs are larger than the gravel pieces: they'd tend to push the pieces of gravel out of place just as you roll over them, which isn't what you want to have happen. And unless the gaps between knobs are the size of the gravel pieces or larger the knobs won't really grab onto the gravel pieces, either. Probably don't hurt much, though, especially if they're more to the side of the tread pattern.
Take into account that you'll always have an upper traction limit of the gravel's friction against the gravel under it, which can be quite low, especially with smooth rounded pebbles that can roll over each other.