Instructions for this image:

1.  Draw gradient.  (Ctrl - J)   Default colors.  VertCenter Gradient.  256 x 256.

2.  Rotate90.  (Ctrl - Alt - X)

3.  Smooth Resize to Width 128 Height 256.   (Ctrl - R)

4.  Copy to Clipboard.  (Ctrl - C)

5.  Undo.  (Ctrl - Z)

6.  Paste as Selection.  (Alt > E > P > S).  Leave at 0,0 and click to set in place.

7.  Mirror L/R.  (Ctrl - L)

8.  Color Rotate setting 8.  (Ctrl - O)

9.  Paste as Selection.  (Alt > E > P > S).  Leave at 0,0 and click to set in place.

10. SineWave Both, settings ud 1 peak 90 height, lr 2 peaks 35 height.  (Ctrl - X)  Repeat three times.

11. Scroll left 128.  (Ctrl - Alt - K)

12. Select the Selection tool from the toolbox.   Select an area starting at X=16 Y=0 and drag to X=240 Y=256 (Width 224 Height 256).   Crop to Selection.

13. Smooth Resize to 256 x 256.  (Ctrl - R)

14. Copy to Buffer.  (Alt > B > C)

15. Mirror L/R.  (Ctrl - L)

16. Merge Buffer 50%.  (Alt > B > M)