Gholdengo's Pokédex number in Scarlet and Violet is 392.
The line is at the very end of the Pokédex, only being proceeded by the legendary Pokémon and some chase Pokémon that are only accessible toward the end of the game. It is very possible that you aren't seeing it because, as it seems, blank dex spots are only revealed when surrounding Pokémon are seen or caught. Check your dex for number 392. If it isn't there try catching a Gimmighoul to see if the missing entry shows up.
Also consider that filled entries themselves only show up under specific circumstances. As Gholdengo is a rare Pokémon, the only way for it to be registered in your Pokédex by name is to obtain one. If you have never caught Gholdengo, the only thing you'll see when looking at its dex number is its habitat, but you should be able to identify the Pokémon by the silhouette provided.
