That sounds like a long-range Kouma lol. But so far Waldstein's c-grabs are around 2k, even his super grab, and they all seem counterable. It's just his Infinite Worth that reaches 4k. For an ultimate move, personally I think he deserves that instant damage if not more.
I too saw Gordeau's c-grab drain that GRD into his own.
I'm thinking that the more GRDs you have, the longer it takes for it to naturally fill up.
It's looking like there isn't any special mechanic for encroaching on the opponent's half of the GRD bar. It's just that if a unit is empty, then either side is free to take it.
But it's not entirely free. It seems that GRD units encroaching on the other side slowly drain away.
As far as I can tell, when you're in GRD Vorpal, your GRD fills up slower. Even when in Concentration, it doesn't seem to fill up any faster.
EDIT: Here's some of the Engrish listed on the character profile on the loading screens:
HYDE - Void Red
His ability, spitting out from the dusky-red aura ****
Grab defensive strength against ****
Officially there is no darkness, only the realization of just this hidden ****
ORIE - Tanatos
Her ability to image of her "innocence embodied in" The ability to summon a magical doll barrel.
Guardian knowing the impurities are innocent, her wish not allow any interference to the desired.
GORDEAU - Bander Snatch
His ability, to realize the hands of his own greed.
Claw-like beast that stretched miss the game and longer
Together with scythe weapon in his "Harvester" has been dubbed with a solid barrel.