Before last night, I used to cringe when a GF joined my team, instead of a rogue, cleric or CW. (GWF are just ambigous, they never die but they dont seem to do much either).
Usually GF's cant hold aggro at all, and chase after enemies that chase after the wizards and clerics, shouting "adds! adds!"
Not last night. There was a GF on my team that held perfect aggro throughout the dungeon (Karrundax). It was only once or twice a random mob got in my or the clerics face, and I was free to blast all the adds to pieces.
So, it is possible to hold aggro as GF, you just aint doing it right.