I think this was painted as an original. Probably not Gimp although should be possible using the mypaint brushes.
You might get something close, blending layers.
A blue base layers
A layer with some clouds, colorized and with a layer mask
A green layer with a layer mask.
Found some cloud images that do not have pintertheft stamped all over; https://opengameart.org/content/fluffy-clouds
...and it goes something like this: https://youtu.be/9I_KcQQZzxw duration 4 minutes.
You might want to try the blue background as a slight gradient blue to a lighter blue. It does become an artistic exercise.
You might get something close, blending layers.
A blue base layers
A layer with some clouds, colorized and with a layer mask
A green layer with a layer mask.
Found some cloud images that do not have pintertheft stamped all over; https://opengameart.org/content/fluffy-clouds
...and it goes something like this: https://youtu.be/9I_KcQQZzxw duration 4 minutes.
You might want to try the blue background as a slight gradient blue to a lighter blue. It does become an artistic exercise.