gear lever? or could it be gear knob?
if you think it is the gear knob rattling and you have a wood aftermarket one, then try tightening the 3 little allen head set screws under that rubber grommet (slide rubber grommet down)
If gear lever, then it could be internal. I would first look under the centre console (good time to check the rubber gaiters are in place and not torn!) and check the breathers have not perhaps loosened...they hold in the shims left and right of the gear lever....I have never seen them loosened, but one never knows.
see 2nd picture here of breathers, you can see the washers and shims left and right of the gearstick mechanism.......
http://www.alfabb.com/bb/forums/1115660-post9.html
could also just be the little pinch bolt that holds the gear lever to that stub........does the gear lever twist at all? (shouldn't!)
you can see the pinch bolt here at the base of the grey metal casting at base of shift lever
http://www.alfabb.com/bb/forums/4688938-post1.html