I will not comment on the Gimp because I prefer other graphics processing programs.
In general, when it comes to changing the background to a solid (white), there are several ways and tools to achieve it, and each has its own one that suits them and has learned.
1) You can create a white layer and overlay this photo and erase the shadow area and the white background covers itself, then you combine the layers together.
2) Another way is to cut the edge of the area in this case of a woman, invert the selection and cut out the background with shadows, or fill it with white paint. The option is a bit time-consuming, but there is a magic stick that automatically marks the area that we want to mark by setting the appropriate tolerance of the selection, i.e. we want the selection to only include a light shade, in this case the white background minus is a white T-shirt which can also be partially selected for this you need to manually mean.
3) Another option is to paint the shadowed areas with a white blurred brush.
I don't know if the gimp is so equipped with helper functions but rather these are the basic options it should have. On Corel PaintShop Pro X6, I usually process photos, I have an XP system, so newer versions do not work for me anyway because they are on a 64bit system. Gimp is partially free, as far as I can tell.
Besides, YT has quite a few video instructions on how to do this.
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