Notes to myself: Using Image>Calculations with Gradient mask

It is possible to create a selective positive/negative image using gradient mask. The resultant image is in B&W with parts of it in positive and part of it in negative.

1. Open an image
2. Create a new file in another tab: File>New. Set Preset to the name of the previous image file.
3. In this blank image, choose and paint in the required gradient in black and white monotone.
4. Go back to the previous image (click on the tab)
5. Select Image>Calculations.Set Source 1 as the original image, and Source 2 as the gradient image. Set Blending mode to Difference, and Opacity to 100%. The result can be set to New Document or as required.
6. To make the image save-able, set Image>Mode to Grayscale, and then repeat and set Image>Mode to RGB colour. The image is now save-able in the correct colour space.