Gear box problem

Hi my scudo 1.9d was stuck in 5th gear for 20min and could not get it out even when turned the van off. It came out of gear some time later and drives ok now but I was told it might just need the gear box oil topping up so when iv had a look at van iv noticed under it that there is oil leaking from gearbox and it seems to be comeing from the drain plug. So ghess my m8 was right and this could be the problem with the gear, s if its losing oil, as I would have to be totaly unlucky if it was the gearbox needing replaceing. Any idea if this is a big job or not? And the cost? As I was told proberbly just needs topping up and a new washer put on? Thanx