get it up on a hoist and check the exhaust for the source of the leak, look at all the flanges and see if any bolts may have come loose causing the 2 pipes to come apart a little and hence leak.
if it's leaking at one of the flanges then reaplce the gasket and put it back together.
otherwise if it's something else like a hole in the system or a crack somewhere then you'll need to replace it.
if you want to replace the turbo gasket that'll involve a little more work and yeh you'll have to get a new gasket from Nissan... I'd also reccomend getting new studs and nuts if your going to pull the turbo off and put it back on.