How to degas FX Royale 400?

Unscrew bottle, if regulated also degas action by loosening the stock support screw (10mm spanner). That's it
Sometimes the reg. plenum bleeds by itself already when loosening the bottle, but that is hard to check. And remember, never dry fire the rifle.

Unscrew the gauge on the rifle until it just starts to bleed air. Leave it alone until it stops. If it's still reading pressure, slowly unscrew it a little bit more. Repeat until empty. 

No problem
Same procedure, but leave the bottle attached ;-) I know there are more roads leading towards the same destination but.....
Instead of unscrewing the gauge, I prefer degassing by loosening the support stud, as a pressure gauge is more prone to breaking, and the support stud seems to be designed to use as degassing screw.
The shaft of the stud is hollow, and there is a bleed port drilled to vent. There is also a o-ring underneath, so the only thing that must be done is retighten it after bleeding.
However, it can be that there is some locktite applied to the thread, it's better to clean it first before putting it back. No need to reapply loctite, I've never seen one come apart whilst taking off the stock. See pics attached.

You learn something new everyday!
I never had a reason to touch the support studs on my Royale's. Thanks for posting this with pictures. I'll start using this method from now on for exactly the reason you mentioned - avoiding possible gauge damage. 
Tom