I thought so. 20 seconds is a long time lol. Please don't take this as a condescending post or anything but we should seriously do a stronghold at some point since we're on the same server! I'm really interested in seeing a GLA DD since I've never actually seen it done at 50.
I guess I never noticed the 20-second debuff, because Riot Blade draws your TP down to zero, and I usually spam my tp-gaining abilities in order to get Riot Blade popped off really quick. By the time you have 1k TP back, there's only a second or two left until you can fire off anyway.

The question wasn't, 'does a GLA make a great DD?' The answer to that question is clearly no, other classes are better suited for it. The question was:

Who has a decent DD gear set up to switch to occasionally when not needed as a tank? What gear are you using? How are you faring against other classes?
This is a good question, because quite often you will end up in parties with another tank (I was fighting in a PT with a MRD the other night and I could. not. keep. hate, without popping every single hate skill constantly. Turns out he had +enimity materia ><) and there's just no need for you to keep your main setup. That being the case, you ought to at least have a backup plan available.

Cons for GLA DD:

- Not that much DMG compared to other DD classes.
- Has to rely on buffs (Raging Strike, Invigorate etc) to keep up.

Pros for GLA DD:

- You can go full-out, constantly, without worrying about the mob starting to attack you much
- You can turn into a backup tank easily to give your main a rest
- You can extra-tank add-ons or links easily
- You can mitigate dmg and take care of yourself, mages don't need to worry about ya pretty much ever.

GLA doesn't make the ideal DD and against top-level players, you're not going to out-damage them. But that doesn't mean you can't still do great dmg and contribute to the group, with the right setup.

Hell, if you had a different set of armor, with the right materia on it, I bet you really COULD do a lot of dmg... I'll have to look into that.