gtk:window-modal (object)

object -- a gtk:window widget
modal -- a boolean whether the window is modal

The accessor for the modal slot gets or sets whether the window is modal and establishes a grab when shown. Modal windows prevent interaction with other windows in the same application. To keep modal dialogs on top of main application windows, use the gtk:window-transient-for function to make the dialog transient for the parent. Most window managers will then disallow lowering the dialog below the parent.