I made some attempts at the Gurney flap simulation, but it looks a little ridiculous to me. JavaFoil shows that the wing at 0 deg perform the same as it did at 12 deg without the flap, but with less efficiency. Link to the first attempt: https://www.imgur.com/LuCvu90
The simulation seems to be very sensitive to the thickness of the flap and the transition to the flap edge. I'm not sure if JavaFoil can simulate the flap properly.
Also I fixed the drag calculation in my previous post.