how many days do i need in the Mutianyu section and how many in the gubeikou section?

Gabe, Mutianyu and Gubeikou sections are a bit far from Beijing. Half a day is enough to visit each section. However, you need to spare a day because you need to go there from Beijing. So two days is ok for your to visit these two places. I think you may hire a car to transport you between these two places because the public transportation betweeen these two sections is not convenient.