GLD as Tank 30+

[Kenka]

You might be able to do higher DPS as a GLD but as a tank, I think Defensive stance will blow any cross abilities out of the water.

This.

You can tank fine up to level 39 sacrificing Sword Oath and Cover, but Shield Oath at level 40 is just too good to ignore. I think that without it you would be obliterated, or at least your healer would have a harder time keeping you up with less max HP and no damage reduction..

[Ehayte]

Norm/Drone here! Any cross-classes you pick up:

1 - Just plain cannot make up for your PLD abilities
2 - Other classes will have them in an 8 man or 4 man party

For instance, you pick up the Lancer 20% slow attack because that should reduce enemy dps right? Well, the SMN/SCH/DRG in your group already has that covered.

I would argue that you probably can do more dps as GLD, but it would be about 75% (at the absolute most) of what any true dps class could accomplish.

Its certainly a fun idea, but SE built the game to make you specialize. However, there could be a dps job released from the GLD class in the future if we are lucky! I'd rather see a Thief > Assassin class though! Duel wield!