What i want no achieve is something like this,
how do i go about defining grids and snap windows to them?
This is generally a feature associated with a tiling WM. Gala is a floating WM and does not support anything more advanced than window snapping.
An alternative would be to 'fake' tiling features using something like Stiler, which lets you bind keys to resize / move windows, emulating the behaviour of a tiling WM.