On my alt I purposefully leveled GLD to 50 without ever changing to PLD just to see if playing as a class made certain content any harder (I leveled WAR to 50 for dungeons, though). While this by no means makes me an expert--especially since I do this for fun, not so much for research--I have a decent understanding for just how much difference there is between classes and jobs. My dungeons experience, unfortunately, doesn't go beyond Brayflox as a GLD, but that's because I quickly saw how much harder it was to hold aggro there when my other three party members were in their jobs. If my party members were all in their classes, or if we actually coordinated better, then I don't think we would have had so much trouble, but as far as tanking goes it's harder to hold enmity against jobs when you are playing as a class. I also ran through Dzemael on my WAR and both DPS were in their classes (I believe it was an ARC and a LNC), and noticed just how much longer each fight against the trash in the dungeon took, as well as the boss fights. I can only imagine what sort of personal hell it would be for the healer if they were just a CNJ, and I don't believe there's any way an ACN would be able to keep up with healing.
That being said, I do enjoy the versatility of the class; I can use Featherfoot to help proc Haymaker, which along with Shield Swipe lets me conserve my TP as well as deal some good damage and inflict Slow. Bloodbath, Mercy Stroke, and Second Wind are always nice to have, as is Invigorate for prolonged fights. I'd imagine Venomous Bite could be useful as well, though I haven't leveled ARC yet. Obviously Cure, Stoneskin, and Physick have their uses, as does Eye for an Eye. If anything having access to all these other skills and spells makes playing a GLD much more enjoyable, haha!
As far as closing the stat-gap between classes and jobs, I'd imagine materia would be the best way to slightly make up for the loss in stats, but I don't believe it would be enough at higher levels. Perhaps fully melded, HQ, iLvl70 crafted gear would be the best route, but since I own no CT gear on my alt I can't say if it is, in fact, better.
Still, I've been meaning see if I can't convince some friends to run Brayflox in only classes, just to see if it's that much harder or not. Heck, if it goes well maybe Qarn or Vigil, too, just for kicks. If anything it'll add a bit of challenge to these old and tired dungeons, and who can complain about that?