I'm confident there are safe images on Gelbooru, but I'd have to filter through all the $%^@ and I don't like that. I activated Safebooru on Danbooru so I don't have to actually use Safebooru itself, but how do I activate safe mode on Gelbooru? Is it even worth trying?

Gelbooru isn't run by Danbooru.

Gelbooru is a whole another site from Danbooru. It just happens to take most of its content from here and keep it with no regards for the quality. (EDIT: Sniped.)

If it's the shit that bothers you, just use Danbooru normally. The quality standards mean the shit is mostly kept away. If it's the unsafe pictures, include rating:s or -rating:e depending on if questionable is okay. Or just use Safebooru.

Gelbooru does have quality guidelines. As for safe images, you can type rating:safe in the tag search at Gelbooru and get only safe rating images for the results.

Also, if you have an account at Gelbooru you have access to a clickable option on your user settings that will automatically black-out everything that isn't rated safe.