Thanks! Much simpler.
Now I’m getting closer, but I still have too figure out how to mask those two buildings at both sides of the image, as well as to give it a smoother transition, especially for the trees on the slope, since a single gradient line doesn’t seem to handle it.
Mask off:
Mask on:
The mask:
UPDATE: As for the left building, I could include it completely inside the mask by adding a second gradient, this time horizontal, and setting its blending mode as intersection:
