As said, check that the breather pipe is not hanging down. It should emerge from the top of the box about 2/3rds of the way back and come forwards and up where it is cable tied to the engine breathers behind the plenum.
Best plan is to mop up what you can - try some degreaser / TFR and then gently hose it off in the hope that once it's clean it will be easier to spot any fresh leakage.
Check the fluid cooler lines between the gearbox and radiator are not leaking first. Then have a look underneath with the engine running. I'd say you should be able to spot it dripping if it's leaking at that rate.
Prime suspects are the output shaft seal at the rear or something in the bellhousing - probably torque converter seal, as with Marie's box.
A gearbox change is not too bad if you can get 2 or 3 pairs of hands on it. We had Marie's car up on ramps and the gearbox out in about an hour and a half, IIRC. Perhaps a bit longer to get it in again because we struggled a little with the TC bolts. Do-able in a day though.
A 2nd hand gearbox can be a bit of a lottery. Depends what's wrong with yours and how much you know of the box you're fitting. If it's a simple seal and you're happy that the gearbox is otherwise fine, I'd change it, but as we found with Marie's, it's not always that easy.
Kevin