The amount of knobs and tread on gravel tyres can be very important for grip off-road, although it is also a big penalty for rolling resistance on-road.
So any way you look at it, depending on the terrain of your gravel ride on a particular day, the tyre choice is important.
In general, the rougher the tyre and the size of the knobs, the higher the aero drag and in particular, the sailing effect is reduced. However in our wind tunnel test, the Continental Terra Speed was a slight exception with a low aero drag level and relatively good sailing effect for its comparatively large sized knobs. There is only a 0.4W difference at 30km/h to the more or less slick Schwalbe G-One Speed tyre. So the Continental Terra Speed tyre would appear to be a good choice in terms of compromise between aero performance and grip on loose surfaces.