After some weeks of some (albeit few) successes, I decided to insulate the downspout from the ground. This was achieved by installing a plastic elbow to separate the metal downspout from the also metal extension resting on the ground while leading outside the yard. The match with the aluminum parts was far from perfect but it was the best available. (see the picture at the bottom).
This seemed to add add an extra S-unit to the signals received. The impression is that the Tx efficiency may have been also similarly enhanced. I have used the miniVNA antenna analyzer on the GU7TR fed via an 4:1 Unun:
This result is not too dissimilar from the one published two years ago by N3VEM for his own version of a gutter antenna as the SWR remained below 3:1 for most of the range of the radio amateur HF bands: https://www.n3vem.com/blog/in-the-gutter.
Since then, a few QRP 20m contacts have been made in the CQ WW WPX CW, several within North America and one with the II9P megastation station in Sicily, near Raguso.