I'll add a little post-script onto the end of SNS3Guppy's generally excellent post.
PT6A free turbine engines are equipped with two controls for the use of the pilot. The first control, commonly called the power lever, is hooked up to a governor that controls the rotational speed of the aft part of the engine (the compressor, or gas generator). The second control, commonly called the propeller lever, is hooked up to a governor that controls the rotational speed of the forward part of the engine (the power section, to which the propeller is directly connected).
The pilot sets these two governors to achieve the desired engine performance. Higher propeller speeds enable greater horsepower to be made, for this reason, the propeller is always set to the highest speed setting for takeoff and landing.
If you observe the performance of the engine closely as you climb, descend, accelerate, or decelerate, you might find that the torque varies even though you never touch the power lever, but the gas generator speed (the rotational speed of the back part of the engine) never varies. The power lever sets the speed of the gas generator, nothing more.
You cannot determine horsepower being produced by looking at any one instrument. Horsepower is a product of both torque and propeller speed. To calculate it, you multiply torque by propeller speed, then divide the result by a constant (K) which represents the efficiency of that particular engine model.