Some people use the term "gear oil" without actually realizing other people think 80w90 when they hear that term.
SUDT2 ... IS... a type of gear-oil. It's just not likely specified by anyone for any product other than Kubota equipment.
And mixing the two different oils was doubtless not a good idea. Different fluids are almost always not to be mixed.
MY Kubota actually says the OPPOSITE. They allow the original Kubota UDT in the front axle to be replaced with 80W90... if desired by the owner.
I think that just adds to the confusion of matters. Kubota should be more explanatory, IMO, as to why they allow 80W90 in the front axle equally rated with UDT or UDT2, for that purpose etc. I suspect it's because the front axle fluid isn't used for hydraulic purposes...only gear/bearing lubrication.
Are you sure you didn't get the instructions backwards yourself?
I'm surprised the gear box didn't have the specified fluid labelled on the unit.
(And while your nephew probably thought he was doing you a favor, the question I'd ask him is, "Why did you not call me first before you decided to do something to my equipt? Would you like me to do that to you? Do you now see why maintenance manuals and specifications are produced?"