Answer:
The zone of which g (gravity) decreases with depth below the Earth's surface is the mantle, which is the layer of Earth in between the crust and the core. The mantle makes up around 84% of the Earth's volume, and extends from around 5 km below the Earth's surface to a depth of 2900 km.
The mantle is composed of solid rock, with the outermost parts of it being composed of more dense, rigid rock. As we go deeper into the mantle, the rock becomes increasingly less dense. The decrease in density along with the increase in pressure creates a decrease in g-force with depth. The decrease in g-force has been measured to continue down to a depth of 2,900 km, at which point the g-force levels off.