Playing as Prussians, I had Gessler commanding a cavalry brigade by february, 1758, when his retirement event comes, and then something weird happens. A "ghost brigade" apperars made up with a mix of cuirassiers and dragoons, and if you put that unit apart from the column, Gessler's picture is still visible, although he is no more present.
Here is a screen:
I have a savegame too, hope it will be helpful.