I have a ruffler foot for my machine which can do everything from slightly ruffled gathers to various sized pleats. Cost about £30
Before the ruffler, to make gathers I would sew two parallel lines with long stitches, knot one end, and then pull the other end to make the size gathers I wanted. I then knotted the other end, and zigzagged over the edge to keep the gathers in place before attaching to the project.