GTR gearboxes are one of the toughest made from the factory and able to withstand 500kW.
But you draw the fine line on how you treat it...
The syncros were made stronger in the R33 GTR gear box but I to have heard that 3rd gear in the R33 can playup.
I would not worry about it too much, as you have to test drive a car and listen out for any 'Crunch' noise.
One scenario I was told was when the cars are shipped on a boat from JAP to AUS, they leave the car in 3rd gear to stop it from rolling in the cargo.
But as the boat rocks, the cars will too, while locked in 3rd gear, causing it to grind.
Don't think it happens anymore.