Ray Prouty says it best:
"No one really understands what happens aft of the rotor mast"
The number of different aerodynamic fixes that have been tried in helicopters to sort out the airflow is amazing, and the AS350/355 family is a case in point.
If you look at the vertical stabilizers on different models of these machines you will see every variation of gurney flaps possible. One side on the top fin, both sides of the top and bottom fin and so on.
Other machines are not much different - I was amazed at the size of the Gurney flap on the port side of the AH-1W fin when I looked closely.
In the same vein, the Gazelle has a simple cambered fin, and it is quite happy to stay at a sideslip angle anywhere plus or minus 5 degrees of zero sideslip in cruise - makes life difficult when you want to maintain a heading. I wanted to put a gurney flap on one to see what it would do, but didn't ever have the opportunity.
The other major use of the Gurney flap is to give a repeatable separation point for airflow. Blunt trailing edges do the same thing (look at the Dauphin or Chinook vertical stab to see this).