Let's say you are standing on the ground and you drop a ball. The ball falls down because of gravity. Gravity is a force that pulls things towards each other, like the earth pulls you towards it because it is very big.
Now imagine you are in a really tall building. If you want to go to the top floor, you can take the stairs, or you can take an elevator. The elevator is like a tiny room that can take you up and down the building without making you tired.
A gravity elevator is like a normal elevator, but instead of using motors and electricity to move, it uses gravity. The elevator is placed in a really tall building, and it goes all the way to the top.
The elevator has two big cables that are connected to it. One cable is attached to the top of the building, and the other is attached to a big weight that is underground. The weight is really heavy, so it pulls the cable down with a lot of force.
When you want to go up, you get into the elevator and press a button. The people in charge of the elevator let go of the cable that is attached to the weight. Since the weight is so heavy, it pulls the cable down really fast, and this pulls the elevator up to the top of the building.
When you want to go down, the people in charge of the elevator let the weight pull the cable down again, and the elevator moves down to the ground.
So, just like when you drop a ball and it falls down because of gravity, the gravity elevator uses gravity to move the elevator up and down a tall building.