I've been trying a few custom maps. But the vast majority of them aren't rendering their assets correctly.

For instance, here are a couple of screenshots of a custom map called SP_GELSEEZE.

https://s9.postimg.org/jixhy5ha7/sp_gelseeze0003.jpg

https://s9.postimg.org/nfatu59zj/sp_gelseeze0004.jpg

How do I get all the assets to render properly?

Comment on their workshop page and tell show them this picture, and tell them to pack their custom assets into the bsp.