I could not get moving an entry in the .ggr file working. Gimp not happy with that 
I thought there must be better way than generating a gradient from an image, and Ofnuts had something up his sleeve. Although editing a palette is easy.
Another way
RobA http://www.silent9.com/incoming/scripts/ Has a script gradient-from-image.scm (look for date 2009-04-09). Very quick.
Does what it says. Then use Layer -> Transform -> Offset to nudge the gradient image along. Repeat gradient-from-image.scm
example: https://i.imgur.com/m4pCu5C.mp4
Might work for you depending on requirements.
I thought there must be better way than generating a gradient from an image, and Ofnuts had something up his sleeve. Although editing a palette is easy.
Another way
RobA http://www.silent9.com/incoming/scripts/ Has a script gradient-from-image.scm (look for date 2009-04-09). Very quick.
Does what it says. Then use Layer -> Transform -> Offset to nudge the gradient image along. Repeat gradient-from-image.scm
example: https://i.imgur.com/m4pCu5C.mp4
Might work for you depending on requirements.