A trim tab is used on a hinged (or torsionally warp-able) airfoil surface to "lever" the 'foil into changing its AOA or camber. As a result, the force generated by the foil increases in the SAME direction that the tab is deflected. E.g., down tab on an elevator levers the elevator surface UP, causing an increase in the elevator's down-load.
The Gurney, IIR, is attached to a non-rotating, rigid foil (such as a race car's spoiler). It adds load in the opposite direction from the direction the flap points. Up-Gurney increases down-load, and vice-versa.
BTW, there's no free lunch. Generating a force by means of an airfoil creates induced drag. The force itself may also add trim drag -- e.g. if the force is a down-load that adds perceived "weight" to the load on the main rotor or wings.
Interestingly, the prop's torque reaction does not, in itself, add to the rotor's load. That's because torque reaction is a pure couple, or twisting tendency. If we counter the couple with a straight-line force in the "down" direction, however, we do add to the rotor's load.
In the same way, the torque reaction from the rotor in a helo does not accelerate the frame in any direction other than rotation about the yaw axis. If we use a tail rotor to counter this torque, however, we're applying a straight-line force that makes the helo want to slip sideways. In contrast, a coaxial helo counters one pure torque with another pure torque. No tendency to slip.
We could counter a gyro's prop torque with counter-rotating propellers and suffer no induced drag or trim drag. But we'd be adding a lot of weight, cost and failure modes for a very small gain in efficiency. Sometimes the cure really is worse than the disease.
