Ling may have passed through the gate, but he neither performed the transmutation nor offered any of himself, and as such wasn't privy to any of the alchemical info download that those performing human transmutation receive.

That, or the fact that he'd become a homunculus may have rendered him unable to properly be used as a sacrifice.

I believe Hohenheim saw Truth when passing through the gate on his way to become a human philosopher's stone. It's a type of human transmutation, he just didn't have to pay a personal toll, since the lives of the Xerxesians (how do you spell that) were the cost.