If you need to erase an image gradually, either to merge photos together, or brush images, here’s how to use the gradient tool to erase. I’m going to use this image of a nice swan..
First duplicate the image onto a new layer: Select All > Copy > Paste. Now select all of the background layer and delete it, leaving you with a blank layer beneath the swan layer.
Click on the layer with the image on it to select it. Now at the bottom of the Layers panel, click on Add Layer Mask
Select the Gradient tool and set it to a black/white gradient, and drag across the image. Now the image fades out to reveal the transparent layer under it.
You can now fill the bottom layer with colour, brushes, or add another image. I added a tree brush on the background layer. (Yeah ok it’s not ever so imaginative but you get the idea!)
And here’s one I made earlier.. This image has three photos merged together – the water at the bottom, the boy on the plinth, and a stormy sky.