Horizontal Stretch
ref image: https://justpaste.it/img/4e69b754fb7cc01f78a9e09342dc1386.jpg
GIMP: http://www.gimp.org
Resynthesizer: http://registry.gimp.org/node/25219
Remember the photographer that recently sold a retouched inspiration for $Millions? Well, I'm going to tell you how you can make one for yourself; my name's Lisle Krolsky by the way (an inside joke that only folks who know me will enjoy; lol). Each of the reduced images can be seen at their respective resolution by clicking on them. :)
1. Take a crop of your target image that you want stretched.
2. Copy the sliver to fill your desired landscape width.
3. Horizontal Blur (high radius).
Below's the result of the blur:
4. Run resysthesizer using the target crop and the blur result with the following settings.
5. Now do some cloning and other enhancements and you get the result below (yes, I re-introduced the tree; lol):
Additional Tweaks:
Same flow above, but if you increase the sensitivity to outliners, you can get a more cleaner result.
result (and again, just a few more clone clean up steps but no where near my last effort):