As it happens I’ve just ordered a new nut together with the packing material for our boat.
Regrettably the nut is not on stock at Amel, they just ordered new ones themselves but it will be September before they are back on stock.
Looking at your picture you seem to have enough room to tighten the nut 1/6 or 1/3 turn.
Be careful as the nut is made of nylon. A rudder gland should not leak at all. The packing material is grease saturated and should keep the water out.
Use a good fitting wrench to tighten the nut and turn it slowly. Do 1/6th of a turn and first assess the result before turning it any more. If movement is extremely hard, the packing material needs to be replaced first.