Visibility gradation between layers?

[CloudyBright]

I want to have two layers - one layer is an unmodified image.
The second layer is a modified layer on top - I want the modified layer to fade to invisible - so the the final merged image will fade from one image to the other.

sunflowers-1716a.jpg

I've done so on the above image by hand - using the background eraser tool and I'm not satisfied with it. Is it possible to do that with a gradient? How?

[LeviFiction]

Masks - masks are layers that hide layers that are underneath them. They are greyscale images. Black color hides, white shows. So you can add a simple black to white gradient to a mask layer, with black at the top and white at the bottom, so the bottom of the layer is shown, and the top is invisible.

Basic steps:
1) Add a mask layer to your top layer. (Layers->Add Mask->Show All)
2) The result is a layer group with the white mask layer on top and the layer it's masking underneath it.
3) Select the mask layer
4) Use either the fill tool or the Gradient Fill tool.
5) Choose B&W gradient
6) Add a gradient to the mask.

EDIT: I've added the two sample images. One shows the layers and one shows the image with the gradient tool active (though the tool isn't easy to see) showing the result of this gradient on the mask.

[CloudyBright]

Okay, I follow but your layer settings seem to have an extra image.
My layers look like this:

settings.jpg

Result was as planned with a background fade to a surrealistic image.

sunflowers-modded.jpg

[LeviFiction]

Mine is a vector so the extra "layer" is the vector object inside the vector layer. But glad it helped.