"齿轮减速装置" can be translated as "gear reduction unit" or "gear speed reducer".
Both expressions are commonly used in mechanical engineering contexts to refer to a device that reduces the rotational speed and increases the torque of an input shaft through the use of gears.
"Gear reduction unit" emphasizes the function of reducing speed.
"Gear speed reducer" is more straightforward in describing its purpose of reducing speed.
You can choose either based on the specific context or preference.