How does gearing affect power generated by a permanent magnet DC motor?
By the principal of conservation of energy, the power output is the power input by the rower minus losses. Mechanically, there are friction losses at each bearing or pivot point, each sliding surface and each area where there is aerodynamic drag. The mechanical power produced by the rower is (force x distance)/time. The mechanical power into the generator is torque x RPM. The electrical power out of the generator is voltage x current. Generator voltage is directly proportional to RPM. Generator current is voltage divided by load resistance. Generator counter torque is proportional to current. Electrical load resistance and losses determine the counter torque felt by the rower.

If the generator torque rating is exceeded, the rated current will be exceed and the electrical losses in the generator will be greater than the generator can dissipate without overheating. The generator can operate at the same current and torque over a wide speed range, but the movement of air inside and through the housing will be reduced at low speed, so it may overheat if there is not a separately powered blower cooling it.