With that definition of truth, a Goedel sentence is not "true", because there are models in which it fails to hold; neither is its negation "true", because there are models in which it does. But that's not the only way in which the word "true" is used about mathematical statements (though perhaps it should be); many people are quite sure that (e.g.) a Goedel sentence for their favourite formalization of arithmetic is either true or false (and by the latter they mean not-true). There's plenty of reason to be skeptical about the sort of Platonism that would guarantee that every statement in the language of (say) Principia Mathematica or ZF is "really" true or false, but it hardly seems reasonable to declare it wrong by definition as you're doing here.