You can use this module with the following in your ~/.xmonad/xmonad.hs:

 import XMonad.Layout.Grid

Then edit your layoutHook by adding the Grid layout:

 myLayouts = Grid ||| Full ||| etc..
 main = xmonad defaultConfig { layoutHook = myLayouts }

For more detailed instructions on editing the layoutHook see:

XMonad.Doc.Extending#Editing_the_layout_hook