There’s a lot of great building advice going around here but I think the original question was, what can the op do to keep that gravel base together? The PT wood perimeter sounds good and would look nice but when I had one of these delivered when I lived in CT, I didn’t use anything. I suppose I could’ve gone with vinyl or metal landscape edging though. For what it’s worth, I was told by the shed company to have only a 6” base of gravel. In the 5 years I had the shed before I moved it was solid as a rock.