Gravity allows you to "set" elements on top of other elements. Instead of manually setting a Z height, the element will be placed at the top elevation of a slab, roof, or mesh, at the clicked point.
These elements can be placed using gravity: Wall, Column, Beam, Object, Lamp, Level Dimension.
I works in plan and in the 3D window.
You tell Archicad what type of element to set on by using the gravity buttons in the Coordinate Box:
L-R: Off, Slab, Roof, Mesh
You can scroll thru the buttons by typing V.
If more than one of the target element type is present at a given point (e.g., floor, counter, and ceiling slabs), the placed element will be set on top of the topmost element. Elements on stories above will be detected too. In order to get gravity to detect lower elements, you can hide the layers of the elements that are "in the way." (Don't forget the Quick Layers palette.)
Tip: Watch the Z field of the Coordinate Box. It will display the elevation that gravity is using at a given point, so you can see if the element is going to be dropped at the right height.
In practice, I only gravity for a couple of things. It's not convenient to use it to place toilets, for example, since they have to sit on the finish floor, and the toilet usually goes in before the finish floor slab. Since gravity always responds to the topmost target element, it takes some planning and care, and it isn't as natural as it could be.
It's very useful for placing 3D trees on a mesh. No matter where you click, you know the tree is going sit on the mesh, and you don't need to know the height.
It's indispensable for associated Level Dimensions.