I'm a bit confused about your problem. You say that you found 'gears', but not 'gearboxes'. That's all you need for calculation. A gearbox is just some gears... in a box. Your ratios work the same way: compare the number of teeth on each gear. The end result is the cumulative interaction. For instance, a gear with 100 teeth driving another with 50 gives a ratio of 2:1. If it's turning at 20 rpm, the driven gear will turn at 40... in the opposite direction. (The torque will essentially be cut in half as well.) Now say the second gear shares a shaft with another 100 tooth piece, which in turn drives another 50 tooth one. Since the ones on the same shaft are turning at the same speed, the final gear will be driven at 80 rpm, with approximately 1/4 of the original torque, and will be turning the same way as the first one. No matter how many gears there are in series, you just keep adding them up like that. The torque doesn't follow the exact ratio because of efficiency losses such as friction, but it's close.
The only other things to worry about in a boxed set of gears are heat control, lubrication, and shifting mechanism (if any).
