At the risk of being flamed I am calling BS so far -
It is not a Gurney flap otherwise it would be at the trailing edge. Its more like a fixed Krueger flap or cuff.
The story I was told is that it softens or makes the stall of the stab less abrupt.
Apparently it used to hang on a bit later than desired and stall abruptly with a sudden pitch change so Panev you could be right with your assumption.
Could also be BS as well but makes more sense than the other explanations.
Someone out knows the real answer.