Sorry but comparing them in combat to each other directly is not a good way of comparison at all.
That's like saying a Slann or Lady Olynder are worse than two Orruk Brutes because the Brutes can kill the Slann or Lady Olynder in melee and they are cheaper.
Or take six Kurnoth Hunters. Sure he will trash them in melee. If he ever reaches them (they have a better move and 30" shooting so they can kite him all day).
Putting models just against each other, trying to bash their faces in, that's not how you should compare balance, it will never work. And some of our units are the best examples for that.
If my enemy brings Gotrek then I just send ten Skinks against him every round. No problem at all. He can never move so he cannot threaten me. Sure he will kill all of them. But my enemy will spend 520 points killing chaff.
The game last night was 1500 points. Sure, Gotrek won the game for me, but he made up more than a third of my army. And it depends on what he fights. He is a monster killer, but against rend immune Nighthaunt horde units for example he is pretty bad. He does kill a lot of them, but they get replenished and 280 points of Chainrasp Horde bind him there for three rounds. Had my opponent not been held up by my summoned Skinks and had Gotrek not been supported by a Carnosaur I would have lost the game.
So I could just as well say that my Slann or my Skinks won me the game.
I am not saying he sucks, but he is of limited use against many units in the game, so very often it might be worth it to instead use some other units for the same point cost.
Well, for a start, when comparing the two Carnosaurs with Gotrek, comparing their combat abilities is the only way you’re going to do that as neither of them can shoot.
As for the Skinks, I still mentioned that they had an obvious advantage in the shooting phase, so no, I wasn’t simply just comparing them in combat. Your example of comparing a Slann to two Brutes is invalid because they have a highly significant points difference - 80 Skinks on the other hand are 560 points, only 40 points more than Gotrek, meaning that they would still have a slight advantage against him but it should be a close-fought thing if the two sides are meant to be balanced.
Concerning the Kurnoth Hunters, firstly they only have a 5” move, only 1 inch more than Gotrek’s, so they won’t be outrunning him by much of a margin (also Gotrek can run while if they want to shoot they won’t be, which balances that out) and the effectiveness of them shooting at him will depend on the deployment distance - if he’s placed 30” away from them, of course that’s going to be an advantage, but if you were to put him, say, 10” away, that’s much more of an advantage for Gotrek. In addition, remember Gotrek’s 3+ Ward Save? That will
Because I believe those two scenarios are highly dependable on such factors, I wouldn’t say Gotrek is as limited as you say he is.
However, GW probably intended for horde units to have an advantage against him in order to again make him balanced. Certainly in Warhammer Fantasy, GW generally designed the rules for monsters in one of two ways to make them more balanced - some monsters were designed to massacre infantry and cavalry units en masse, but these struggled against other monsters, while others were designed expressly to be monster killers but could be bogged down by infantry - Gotrek was thus designed to be one of the latter type units.
Besides, the horde units that seem to have an advantage against Gotrek struggle to do anything against the massive deathstar characters like Nagash and Archaon that have become more commonplace since AoS arrived, which in turn are easy prey for Gotrek, meaning essentially he’s been designed to be the last piece in a rock-paper-scissors puzzle where each thing has a speciality against something else but has its own weaknesses.
Essentially what I’m saying is that Gotrek was designed to walk all over things that before now had relatively few counters, even if he has a commonplace counter himself, which essentially is there to stop him from becoming ridiculous. It just so happened that in your game you were up against one of those armies with more counters available to them. I imagine against other armies he’ll be a lot better, and I think you should try using him against other factions before you dress him down too much.