What is the gearing bonus on Hearts Of Iron 2 Armageddon

The Gearing bonus reduces the TIME it takes to build units or upgrades if you build them consecutively 'in series' so producing 20 identical infantry divisions, starting one after the last is finished will give you a gearing bonus as your factories specialise in it. Immovable province improvements do not get a gearing bonus, only movable units or upgrades do, the IC used for production also stays the same, your units will just be produced quicker.