According to conventional wing wisdom it really should not work!
But this is what happens [sometimes] when you deal with hot rodders!
A lot of research went into this in the 80's and 90's. If we put a Gurney flap on a wing trailing edge and then fair the upstream side we create an artifact called "trailing edge bluntness". It was actually pioneered in the 60's by Whitcomb. The drag of an airfoil is actually reduced by having a little of this depending on Reynolds number: the higher the Rn, the thicker the boundary layer is and the more bluntness works for us.
Essentially, it delays the closure of the airfoil trailing edge upper and lower boundary layers until farther aft of the physical trailing edge. So we get the benefit of an aerodynamically longer chord without the drag of longer chord, at the expense of some trim drag due to higher negative wing moment. It also can make for a lighter structure, and weight is everything for an aircraft. The most dramatic example currently is the G-650.
It takes pretty high end CFD to accurately model this effect and a lot tighter mesh than most people are willing to run. It's not magic, it is well known aero for 30-40 years that is in wide use.
As for un-faired Gurney flaps, they are in use on many small aircraft to stiffen the trailing edge of control surfaces. T shaped flaps on the rudder of many small aircraft prevent flutter and unload the control forces at minimal drag penalty. Conversely, many composite kit aircraft use way too much trailing edge bluntness because the bondo jockey who built it got lazy. For under 10M Rn, 0.5% chord is about right.