CR 6 I guess. They're a little better than a girallon but not as good as a hill giant, I think.
My initial gut reaction is Gladiator Lizards are more like Challenge Rating 5.
Combat wise they've really only got their unusually high Armour Class going for them.
Potential damage output is much worse than a Girallon: 2d10+16 vs 10d4+1d8+36, or 27 vs 65½ if we ignore the odds of a critical. That's 242.59% the damage of a Gladiator Lizard.
The Lizard does hit more often with its +13 melee though, or +15 with Hive Mind and a double-teaming brood mate.
So is 6 points more AC and a bit higher odds to hit equivalent to 140% more damage?
Hmm, they would approximately balance if a typical melee opponent had a 25% chance of hitting the Lizard, as they'd then have a 55% chance of hitting a Girallon for 1/2.2 times the DPR. That'd mean a hit roll of 16+ or an opponent with +7 to hit. That is a slightly low for a 6th-level combat specialist though.
Well their various fighter feats give them ways to boost their DPR using Combat Reflexes or Cleave to get an extra attack or two, and they can Power Attack soft targets to raise the damage.
Guess that
could nudge them over the line to Challenge Rating 6.
How about a
Large Earth Elemental as a comparison? That's CR 5. A Large Gnome has similar HP, is pretty tough (AC 18 and DR 5 might eyeball as roughly equivalent to AC 23) and does slightly more damage (4d8+14 for 32 on average) with a bit less accuracy (+12 melee).
It has much worse saves than a Gladiator Lizard though, which should count for something.
Hmm… having trouble deciding.
They could be a weak CR 6 or a strong CR 5.