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Do not confuse torque and power.
If you plan to do heavy cuts below about 1200 RPM, then get the gear box. Around 1200 RPM is the usually setting, but it is changeable, for shifting the gear box.
For a motor producing 1 HP at its ouput shaft at rated speed the output torque is
T = HP * 5252 / RPM in #-ft = 5252/RPM .
If the motor power output is held constant and you add a gear reducer, then the power out of the reduce will be slightly less than the 1 HP from the motor, but the torque will be higher than at the motor shaft.
Note: most electical motors can be consider as constant torque limited vs speed because torque is proportional to current, and largely the heat losses in a motor are I*I*R. Internal power dissipation inside a motor determines its rating. A gear box or belt reducer with relatively high efficiency increases the torque but lowers the speed.
If you use a 1/16" endmill you do not need the gear box, a 3" facemill will require a gearbox.