Once you enter the great gulf nothing can stop your fall into the core. The pit is everything below the mantle. If the sides of the pit is the lower mantle, then the pit includes the great gulf.
Sides: בְּיַרְכְּתֵי – beyerekah
The Sides of the Pit
We (that's me and the Holy spirit) have been working towards the construction of a model of the interior of the Earth which fully integrates every possible way that hell or the afterlife is referred to in the Bible.
March 20th
If you're on a downward course and leave the lower mantle, once you enter the great gulf, there's nothing to stop your fall into the core. This means that the pit refers to everything below the mantle.