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This Gymnosoma was nudifrons after all. I send the fly to Zeegers who had kindness to look closer at it. This is his answer:

" I had a second good look.
As said, based on the frons, your specimen is intermediate.

So we have to try other features. These are only 90 % reliable. But if
you combine these (totally dark scutellum, darkened wing, form of
genitalia), all point towards nudifrons.
SO, I am inclined to think that this is an aberrant female of G. nudifrons.
In any case, it cannot be considered a definitive (or probable)
rotundatum. So, false alarm, after all, but it is not weird that I was
misled by the pictures !"