In GIMP, add a layer mask (white, full opacity), add gradient with black FG and white BG color, 100% opacity
If Mode:'Normal', pull from about 60 to 120, maybe better from 60 to 100 for more steep grad
If Mode:'Hard Light', pull from about 60 to 150, maybe 60 to 120
Note dimensions 128x40000, cut should be at around -100 from the top

-Normal 60 to 120 is slightly longer and slower fade, try 80-120 or 80-110 (80-110 is good fade point but too fast fade)

-Hard light 60-140 pretty good, maybe 70-150 or 60-150/160 even better

- Linear burn 25-115 (110 also nice) (or possibly 0-100)