Very nice picture indeed. Just a note about the watermark filter of G'MIC :
If you don't customize the watermark parameters too much (e.g. orientation of the text), this is actually very easy to remove the watermak, using the same G'MIC Watermark filter : just set the actual text of the watermark, set mode to 'Darker' instead of 'Lighter', lower the opacity, and you're done. Obviously, I won't show the entire result here :p, but I managed to removed it quite easily on the high-definition image (just show a crop of my result below)
So, be careful if you really want to get a durable watermark, you should at least modify the default 'Size', 'Angle' and 'Smoothness' parameters of the filter, even a little bit, so that the watermark becomes harder to remove.