How can Solemn Judgment negate Gorz the Emissary of Darkness but not Chaos Sorcerer in Yu-Gi-Oh?

Solemn Judgment can negate summons that don't use the chain, by waiting until the monster is placed on the field, then negating the summon before it counts as being successful. When a summon occurs on the resolution of an activated effect, the summon is automatically successful when the effect resolves - but Solemn Judgment can chain to that activation and prevent the summon by stopping the effect resolution.

So, if you Normal Summon, Synchro Summon, or perform a special summon that does not use the chain, such as Chaos Sorcerer, you can negate the summon as per the first bit above.

If you special summon on resolution of an activated effect, such as Monster Reborn or Call of the Haunted, Solemn Judgment can chain to these activations and negate them. No resolution means no summon.

But, note that Solemn Judgment can only chain to the activation of Spell or Trap cards. If the special summon occurs on the resolution of an activated monster effect, Solemn Judgment can't stop it. It can't chain to it, and as above, the summon is successful when the effect resolves.

So that is why Solemn Judgment can't do anything against the triggered effect of Gorz the Emissary of Darkness, but is able to stop special summons that aren't by resolution, such as Chaos Sorcerer.