The Grand Company barding can only be used if you're a member of that actual Grand Company - if you switch companies you're forbidden to use another Company's barding until you change back to it (for instance, if you signed up with the Maelstrom and then switched to the Twin Adders, like your screenshot seems to suggest, you cannot equip the Maelstrom barding - you'll only be allowed to equip Twin Adder barding). I'm pretty sure the Active Help actually mentioned this when you first purchase company barding.