Hi [@Burthstone]! Non-humans can be sent through the gate: usually in the case where the humans don't know they are non-human (or, perhaps, one official person might twig and send them off in a human's place). Sometimes, magickal creatures raised in the human realms decide by themselves to go and join the Tolls of their own volition, claiming to be one. Magickal creatures can be born in the human world as well, such as changelings or creatures born of human emotion. 

Also when the Gate That Never Was was created, supernatural creatures escaped back into the human realm. These days they are not in the same numbers they were previously, prior and during the Dark Ages, though, and humans and magickal creatures struck a deal to stop them coming back and murdering everyone senselessly (which they did for a very, very long time). But magickal creatures pretty much do what they want for the most part these days. They try to keep it low key though, because they get their Tolls, and they don't want humans rising up against them again, so there's kind of this uneasy but somewhat comfortable "peace" for everyone.

Unless you're a human living beyond the Gate. That's... yeah that's not peaceful. But everyone else is great! :D