3000*0.477 = 1431
Rocket science it ain't...
The convention is that the gear ratio is output:input. The maintenance of this convention obviates the need to say "reduction" or "step-up"; but doing so provides an additional check. The same sort of logic applies to the Pressure Ratio of a gas turbine compressor (such that if you want a nice number >1 when considering turbine performance, you'd call it Expansion Ratio instead).
Now, since the above answer is exact, you may be wondering why I said "about" 1431 rpm. Well, there are several reasons. Firstly, I haven't counted the teeth so I don't know if 0.477 is exact or whether it's an approximation. Secondly this whole business is somewhat approximate anyway; I don't know how accurate the rpm measurement would be, and it doesn't make any difference to the argument, so why worry?
You also don't need a torque curve to explain the fact that rpm falls when pitch is coarsened.
Blade alpha increases, therefore blade CL and CD increase. The power required to drive the prop is larger than the power supplied (since input power hasn't changed, and the system was in equilibrium before).
However, the force on the blade is proportional to the square of the tip speed; thus the power required is proportional to the cube of the tip speed.
At constant engine torque (i.e. roughly constant BMEP) the engine power varies directly with rpm.
Therefore as the prop slows down its power demand falls much faster than the engine power output and so a new equilibrium rpm is reached.
No torque curve required.
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