Not 100% sure, but I would say that the length and markings on the oil dipstick will be different to the gearbox dipstick.
What you should do is take your oil dipstick to the shop, and before buying the gearbox dipstick, compare the 2 lengths and the markings. If they are the same then do not buy it.
Otherwise if you really want to save a buck, what you can do is go to the shop with a brass wire and a hacksaw. Bend one end into a circle (for the handle), then cut the wire to the exact length of the gearbox dipstick and use the hacksaw to make a Min and Max marking.... voila... you've made your own dipstick