If you just have a gentle slope, you aren’t going to see much definition in your render. If you only use GI, without an HDR texture used for GI, or the Physical Sun/Sky, you won’t get much definition in the render, because everything will just sort of be lit in an average way. You need to either have detail in your model so Ambient Occlusion, as dcauldwell metioned, can help define edges better, or you need to have more complex lighting. Adding a reflection layer can help some, but it won’t get the clay render you’re looking for, that’s going to be achieved through proper lighting and/or Ambient Occlusion.