what class can wield a 2h sword ?

[Enkidoh]

Probably the closest GLA/PLD weapon to a two-handed sword is the PLD relic weapon Curtana, due to it's size, but even that's wielded in one hand. Single-handed swords are the weapon type for GLA and PLD.

As has been said, 2-handed swords do not currently exist in FFXIV. It's possible it might be brought in at a later date (especially as early cg artwork for version 1.0 showed 'Derplander' with a 2-handed sword strapped to his back), but nothing is known for certain at this stage.

As for GLA being a damage dealer, it is possible, but the official designated role for GLA (which it's job upgrade PLD builds on) is tank. PLD does get an ability called 'Sword Oath' which reduces defense but increases damage, but this is more meant for soloing.

Having said that, some party situations where there are multiple tanks in the party you may in fact be asked to DD (with Sword Oath), but usually you'll be expected to tank (with Shield Oath instead). It's the nature of the beast.