Gathering is a sewing technique that reduces the length of piece of fabric, so a longer piece can match and be attached to a shorter one. It is used yo manage, as much as disguise, a source of fullness- such as on a cuff or sleeve - and can also pinch a skirt into a waistband or bodice.
Pleats are a fold or doubling of fabric that is pressed, ironed or creased into a place. They add an even greater fullness to a shape.