A gearbox on its own won't really regulate speed, apart from the fact that it will have some internal friction but this is difficult to regulate. For example just having a 1000:1 gear ration won't make the object fall 1000 times slower unless there is some external torque applied.
A simple solution is to use viscous drag for example having the gearbox drive a fan or paddle wheel. The works because viscous drag increases with the square of speed so you will get to a point where the drag force equals the weight of the object at some reasonably constant speed.
There are also various types of mechanical governor which can be used to adjust the force on a mechanical brake in response to speed. A step further is to use an electronic system with speed sensors and actuators on a brake, this potentially allows a lot of flexibility and the ability to self adjust on the fly.
Another option is to use an escapement of they type used in clocks. This doesn't give true constant speed as the motion becomes a series of discrete steps but for the very low speeds you are talking about this may not be a problem.
Having said that the gear ratios you are talking about are huge, to put it into perspective if you did this with a single pair of gears and the driving gear was 5cm in diameter then the driven gear would need to be 5km in diameter
Edit in response to comment:
If you want to use this arrangement to generate power then you need to bear in mind that gravitation potential energy has pretty poor energy density compared to other small to medium scale energy storage.
Specifically the total gravitational potential energy is 9.8J per kg per metre of drop. 24 hours is 86400 seconds so for a 2m drop this translates as 1.1e-4 Watts ie you would need 5000kg to provide 1W over 24 hours before considering losses.