The correct answer is B) On the accessory gearbox. The gas generator speed sensor is typically located on the accessory gearbox of a marine gas turbine engine. The accessory gearbox contains the sensors and instrumentation necessary to monitor the engine's critical operating parameters, including the gas generator speed. This location allows the sensor to accurately measure the rotational speed of the gas generator, which is a key indicator of the engine's performance and operating condition. The other answer choices are incorrect because the gas generator speed sensor would not be located at the compressor inlet (A), within the high pressure turbine (C), or within the compressor (D). These areas are not typically accessible for sensor installation, and would not provide the necessary data for monitoring the gas generator's speed.