Appearance – Widgets – sidebar – gallery
click inside the group of pictures (not ‘add media’)
click pencil to edit
then ‘add to gallery’ on left side
click ‘update’ at top’ to save the widget

Appearance – Widgets – sidebar – gallery
click inside the group of pictures (not ‘add media’)
click pencil to edit
then ‘add to gallery’ on left side
click ‘update’ at top’ to save the widget