That's an interesting question. I, too, could not find any interval for this in the owner's manual. The longest interval for anything seems to be "Check clutch (clutch removed)" at 24,000-mile intervals.
I had my '22 C 400 GT long enough for the initial break-in service and the 6,000-mile service to be performed. Both of those were done at Max BMW ((the Troy, NY, shop), and there was no gearbox-oil change listed (and they seemed to list everything else they did, and there's certainly an incentive for a shop to do as much as possible, as that's how they make a lot of money).
(BTW, I'm 75 years old, and have decided that I'm not doing any maintenance myself, if you were wondering. I'm fortunate enough to be able to afford to pay the dealer to do this. I like spirited riding, but I don't like getting down on the garage floor. And I still do electrical farkles on bikes, as I enjoy that, and it's mostly stand-up work.)
So, it looks as if your dealer and mine agree that gearbox oil changes are not part of routine service ... unless they do it at some very long interval. It's reasonable for the shop manual to include procedures that are not part of routine service, just to cover situations in which parts are replaced because of failure, accidents, etc.
