I think I solved last night the mystery of my Gentiana. I have been in Summer 2015 in Denmark's island Roemoe.
Such Gentiana were in flower there.
No seeds so I did not think of them at later times. But I did collect some small pieces of moss and some feces with a lot of berry seeds from the bogs there.
I must have put some of it into my existing planters. So might at least two of the small Gentiana seeds might have travelled there unnoticed.
The planters where I put most of it have no Gentiana plants growing. Only some of the berries.
This is a picture from my holiday, it seems to be the same Gentiana, though not exactly the beautiful color of my own plant.