blending backgrounds

[df23]

I have an image and i need to replicate part of the image and put this below the original image to create a longer image. I have done this by creating layers for the original and the partial copy. The background colour (in this case the sky) at the bottom of the original and the top of the copy are not quite the same. I need to be able to blur the sky colours together. I cant use the gradient because there are other parts in the image (ie not the sky) at the border between the layers which would be impacted.

Any suggestions on how this can be done?

thanks
Dave
Gimp 2.10