Originally Posted by Wirespokes
I've seen that sort of wear when the front output shaft bearing has gone, and the gears don't mesh properly, along with grinding all the swarf floating around in there. It might not hurt to re-use the gear, I don't know. But if another better one was available, I'd certainly change it out!
It's actually best to use fifth as an over-drive and not put much stress on it because of that front output bearing.
I would absolutely replace the gear. Gears are case-hardended and have a relatively soft core. Damage accelerates once you wear though the hard surface layer, which that gear has.