gtk:box-center-widget (box)

box -- a gtk:box container widget
widget -- a gtk:widget child widget to center

The gtk:box-center-widget function retrieves the center widget of the box. The (setf gtk:box-center-widget) function sets a center widget. That is a child widget that will be centered with respect to the full width of the box, even if the children at either side take up different amounts of space.