^Even still, it's not just a matter of G&W's ability to be able to perfectly space. It's also about marth letting G&W get to that exact spacing. At the point where G&W is always at the absolute reach with his bair, Marth can take take a few steps back and out of the way, then ftilt. If G&W is still bair'ing and DI'ing toward you, shield because he won't always be able to predict where that exact spacing will be.
Then there's the whole option of just countering =b I think pierce said it before but counter can **** some of G&W's move-set pretty hard.