This article explains how to recreate the Orton effect, an old darkroom technique used to add a surreal soft-focus filter to photographs, using GIMP. The process involves sandwiching multiple versions of the same image on different layers. The instructions in this article apply to GIMP version 2.10 for Windows, Mac, and Linux.
Tutorial: How to Create a Soft and Dreamy Effect in GIMP
To produce the Orton effect in GIMP:
Open your photo in GIMP and select Layer > Duplicate Layer.
Make sure the top layer is selected in the Layers panel, then go to Filters > Blur > Gaussian Blur.