gtk:grid-insert-next-to (grid sibling side)

grid -- a gtk:grid widget
sibling -- a gtk:widget sibling widget of the grid that the new row or column will be placed next to
side -- a gtk:position-type value for the side of sibling that child is positioned next to

Inserts a row or column at the specified position. The new row or column is placed next to sibling, on the side determined by side. If the side argument is :top or :bottom, a row is inserted. If the side argument is :left or :right, a column is inserted.