Nissan Repair Questions? Ask a Mechanic for Answers ASAP
More than likely if the new gear box is leaking oil its at one of the lines to the cooler. it's not uncommon for a cooler line to get damaged upon removal and then need replaced so the shop should have payed close attention to that and inspected for leaks afte a test drive.
I would bet it's something that was overlooked and not an issue directly related to a gasket or anything of that nature.