For any of Gravel (as for Lawns and Meadows) preset you can find two versions, one with a "Detail" suffix and another with a "Large Area" suffix. Looks like, in your scene, the "Detail" one would suit better. Also, I would suggest you to check the Direction settings under Surface tab (probably is set to 100). Setting it to Surface's Normal (0) will help on sloped surfaces. Also, specially for these purposes, you can take advantage of Scale to fit sloped area option. Hope that helps.