Since discovered the idea to merge multiple images, and found a simple way to use gradient selection, I found new interesting use for both of these tricks.
In this tutorial I used two different images. On first one there is just a single tree against snow, and two trees on another. First I just open both using 'File > Open as layers...' on GIMP (hold Ctrl key down when selecting multiple images). Then I changed the layer mode to 'Darken only' for the upper layer (image with two trees).
Now comes the tricky part. Follow the instructions on my previous tutorial but instead of using 'Radial gradient', keep the default 'Linear gradient' to select half of the image. When done you should get dotted selection line that vertically split the image from the middle. Now you can use 'Select > Invert' to toggle between left and right side of the image.
In this example I first used left side selection for image with two trees to blur left side of it 10 pixels 'Filters > Blur > Gaussian blur...'. Then I used 'Colors > Curves' to brighten the left side of the image a little.
Next I changed the selection for right side of the two trees image. For that side of the image I gave heavier Gaussian blur (20 pixels), and made it even lighter with Curves. As a result I got image with three trees where one of them looks sharp on the front middle of the image, another tree slightly blurred on left, and more heavily blurred tree on the right. A clever way to create shallow depth of field look on the image, I think. And also a clever way to adjust brightness only for selected part of the image.