Basing on my experience, GG with shields and full ranks are really tough against other infantry, and they break cavalry, even chaos, due to CR. Killing blow is deadly, and I once broke chosen chaos knights of tzeentch in 2nd turn, who had charged me, but then I had the cursed book nearby. What GG cannot handle, are trolls, minotaurs, dragon ogres, treeman and the like. Now, I think the new swordmasters might be a match for them but they should handle new dragon princes.
Heavy shooting is a bane of our kind anyway, of course the units that cannot be replenished suffer more, but in my personal experience the enemy almost never shot at GG. Why, I think he had other, more juicy targets. First, the unit with general or another with BSB, then BC if I had one, then the fell bats and wolves (who were hunting for war machines). I'd say GG should not be afraid of S3 shooting.
The main drawback of GG is their slow movement, and the need for 5 in rank to keep the maximum bonus in 7th ed.
The other is that it takes up one precious Special slot.
This is a unit that should hold the centre of battlelines, but must be protected against being flanked. If you play infantry, they are a good breaking unit, which does not need characters to win combats, but if you play cavalry, a strong unit of BK is better. To some degree, mixing is possible, and for me worked one BK unit with characters plus one GG unit. My idea for this combination was simple: anvil for GG and hammer for BK.