I am asked quite a bit what is the difference between the double shirring foot (left) and the gathering foot (right). Both give a slight gather to your fabric. The sheerer your fabric, the tighter the gathers. Both will gather and both will shirr fabrics. Shirring would be two or more rows of gathers sewn closely together.
The difference is in the design of the double shirring/gathering foot. Note the slit in the side of the foot. This slit lets you gather and sew onto a second fabric at the same time; in one sewing pass. On the left is a vintage double shirring foot. New ones (right) are readily available quite inexpensively. You can purchase low, slant, high, or snap-on shank to fit your specific machine.
Slide the second piece of fabric into this slit, place your piece to be gathered under the foot. Begin sewing. While the bottom fabric will gather, the top does not. Cuts your sewing time!
If you just wish to gather one fabric, your double shirring/gathering foot works nicely. So, you have the option of gathering one fabric alone, or gathering while sewing to another.
A gathering foot merely gathers, but does not sew onto another fabric at the same time. Again, the sheerer the fabric, the fuller the gathers. When gathering 100% cotton fabric, I can use my gathering foot to gather. Leave a 6 inch tail at the end. Grab the top thread and pull gently. This will gather the fabric some more. Continue pulling until you have the amount of gathers you wish. Using a heavier thread in the top gives the threads more strength and no fear of breaking when you gently pull the thread.
Check yours and see whether you have a gathering foot or have a double shirring foot. Pull it out and practice on various fabric types. Lengthen and shorten your stitch length. Just practice and have fun! See what it will do.
Now, what about the differences between the gathering foot, the double shirring foot, and the ruffling foot? Another time!