Gloomspite Gits will have a field day against them in the larger games. Not necessarily because of any of the trolls, squigs, or other monsters in their roster, but rather because of the exact thing that those gargants are supposed to be good at dealing with - massed infantry. A skilled GsG player is likely going to make sure that the front line of their night goblin spearmen units have at least one netter every 2 inches or so by default, ensuring that whoever attacks them from the front is at a constant -1 penalty to hit them, and if properly supported (as they should be) those lowly spearmen have both a lot of staying power as an anvil unit and can hit back harder than you'd expect out of stunty greenskins. Moon-Face Mommet is also nothing to sneeze at either.
Similarly, I'd expect Beasts of Chaos, especially Khornate Beastmen, to make mincemeat out of the gargants with massed infantry charges from across the board, especially if supported by the use of a Wildfire Taurus (D6 mortal wounds to units within range of path + units affected activate last in combat that turn).
Mind you, I'm largely drawing this up from experience within my local gaming group, where the recurring theme tends to be making casual "take all-comers" lists feel incredibly competitive.
but that's a matter of personal preference when it comes down to it 
. Or can be viewed as primaris gargants?
