I think Aero for gravel is rather silly, but if you are looking at aero for the road portion of your ride that makes sense.
Aero really starts to make a difference over 20mph. Below 15 it isn't much of a factor. Generally I'm not doing over 15 on gravel solo (but sometimes in a pack, and always for that sprint).
I recently got some 3T wheels. Aero wise seem to do pretty good in a 20-30mph range (including apparent wind) with slicks. Any knobs is likely to ruin the air flow over the wheels. Then again, it may just be the placebo effect.