I've been riding grass, gravel, packed lime, aglime, macadam, asphalt, and cement my whole life - and you can expect any tire to lose traction in loose gravel - and, I just deal with it.

I find my Schwalbe Marathon Supremes, 700-35 and 700-40, adequate for all of this. And, being a semi slick, they still behave perfectly on pavement.

Buy the tire for it's most used surfaces, and you should be pleased.

I love the combo of a 700X40 on the rear, and a 700X35 on the front - I'd also consider a 700X32 for the front; but don't know if I'd be willing to give up the comfort of the 40 on the rear.

p.s. my bike came with 45s as OEM, and don't notice any different comfort with going down to the 40 on the rear. Nor, do I notice any degradation of comfort, with the 35 on the front - but ---- the 35 sure quickened up the steering, and improved the handling immensely. The 35 did ride quite a bit harsher on the rear.