You start at 0 GP after dying because if you were to get GP when revived, you'd could kill yourself for GP to restore GP and you wouldn't have to use cordials.

The reason you start 400 GP after switching from another job is because all classes have 400 GP, and it regenerates on all jobs. It can't regenerate past 400 because that's the cap without equipment. SE could set the GP to something like 1000 on crafters and combat jobs, then it would go to up to 1000. Then when you switch to a gathering job, it would reduce down to 600 GP or whatever your gather job have for GP.