Gearboxes are basically exactly the same as sprockets but they gear by the teeth of a cog binding together. The gearing works on exactly the same principle as the sprockets.

A gearbox is just a housing for gears. Gearboxes can have 1 or many stages of reduction, dependant on the motor and gearing required.

They can be made from scratch and/or scrounged (bit harder) but for a 24v gold you could easily use sprockets or timing pulleys to gear the motor down.