The depressurization of the landing gear hydraulic suppy at speeds above 280 kts, is a design feature to eliminate the need for a gear lever "neutral" position.
I'm certain that there is, in fact, electronic logic to prevent activation of the LGCIU below 21000, but I'll be dambed if I can find a reference to it.
However, I've got drinkin' plans with our local Airbus Tech Service rep tonight, so I'll throw the question at him and let you know later.
Cheers