[MrForbes]

You can still define the requirement for the gearbox by using speed. Then you can figure out what ratio you need.

Say, you want your arm to move 90 degrees (from horizontal to vertical, for example) in about one second. That's 0.25 revolutions per second, or 15 revolutions per minute. An RS775 motor has a speed at it's "sweet spot"of about 15,000 rpm. That means you need to have about 1000:1 reduction to have the arm travel at the speed.

I'm making a lot of assumptions about your robot design, since you provided no details yourself. If you want more useful suggestions, you really need to provide us with a bunch more information about your robot.