No idea what gear box or pin you are referring to, need to be a bit more clear on that but if it is the one under the clutch housing and looks like the end of a cotter key then it there to rattle around and keep the hole open so water, oil or whatever can drip out of the housing. A simple way to keep fluid from building up in the housing to damage the clutch.
If that is not what you are talking about post a pic.
