In other tutorials on the blog you can check out how to create a bias tape using your own fabric. A good option for neatening up a seam with gathering would be to cover the seam with a bias tape of the same fabric as the one you are using to sew the garment itself.
If the fabric is thick you can opt to choose a lining fabric in a similar shade color. The lining is a very thin and manageable fabric so it is also a perfect choice for the visual finish is not coarse. You could also use a contrasting color to create a nice play of colors on the inside of your garment.
Already have your bias tape? The first thing to do is to cut any frayed threads from the fabric.
Then pin the bias along the seam allowance. I like to rest the bias on top of the gathered piece but it’s just a personal taste.
Topstitch at presser foot distance from the edge.
Turn the bias over to the other side of the seam to binding the gathering completely. Hide the seam allowance on the other side inside the tape. At this point you have two options, once all the bias is pinned or basted you can sew it with another machine stitch or hand stitch.
And in this simple way the inside of your garment will look beautiful and with a quality finish without the need to line the garment.
See you soon!