I've never lined my beds and I've never noticed any soil leaching out. However, I dug a shallow trench around the outer edge of the beds and place the boards into it. I don't know about a gravel base. I suppose it would depend on how dense the gravel is? If it's loose it'd just be like a base to the bed, but if it's hard packed then you might have an issue with water and soil run-off.
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