Gearbox in neutral otherwise the parking brake won't release, not sure if there's a manual cable like they have on Land Rovers to emergency release. If you have totally lost all electrical power to the vehicle then there is a bolt you can screw in on the bottom of the gearbox to release the parking brake so it can be towed.
There's a pretty digestible write up about it
here.
One thing I'm not sure on is if INEOS are changing out the standard plastic oil pan / oelwanne of the ZF box to an alloy or steel version. The plastic pan has an integrated filter assembly for the transmission fluid. As far as I'm aware the steel versions have a separate filter assembly. I forgot to look with my excitement when I saw the Grenadier in LA.
For reference Land Rover use the ZF plastic pan with additional underbody shielding to protect it.
As far as LR are concerned the transmission fluid is for life, but ZF say 70 or 75K if I remember correctly for a drain and refill.
Here are some photos of the new pan ready to go on my Disco4/LR4
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