The answer to that question depends on what you're using the concrete for. If you're using it for a driveway, then the answer is yes, you need gravel under the concrete. If you're using it for a sidewalk, then the answer is no, you don't need gravel under the concrete.

Generally speaking, no. You don't need gravel under concrete unless you're using the concrete for a driveway or some other application where vehicles will be driving over it. In that case, the weight of the vehicles will compact the soil underneath and create a stable surface for the concrete to rest on. If you're just using the concrete for a patio or some other non-vehicular application, then you can get away with using just sand or another type of aggregate as a base. But if you're going to be doing any heavy work on the surface (like drilling or hammering), then it's always better to have a layer of gravel underneath to prevent damage to the underlying soil.