That's the hard part. You need to know what props they were supposed to be.
The only way to find out is check what dependencies a Gmod map has, and either buy the game for its content, or find some place to get its content.

Usually the mapper should include all required content though. but rarely anyone ever does that.

ok but HOW!?

Those are missing models. They can only be fixed by adding the models back into the game/addon.

what about the big red texts?

nice

Destroyer of the purple checkerboards! Nice! :D