OK, this is my expertise! I regularly fight with a very good G&W.
The first thing to remember is that Bair has a landing hitbox. When G&W touches the ground, even if the move appears to be finished, an extra hitbox pops out. So remember to hold your shield for longer than usual! Unfortunately, as a result of this, it's extremely difficult to punish because you get pushed back when the landing hitbox hits your shield.
There's several options for punishing turtle, but only one of them is consistent for any setup and it requires forethought, so it's vital to be prepared for any situation.
1) If he spaces Bair properly horizontally to prevent you from punishing it OOS, you can ftilt him as he comes in. The timing is very close, but it works!
2) If he starts spacing bair high, you can utilt or even up-B I think. I usually just roll back, or run into it with shield so I can dash attack him afterwards because he's too close.
3) If he dives too deep into your shield you can shield grab him just before he hits the ground. Don't even bother trying unless you have the timing down really well.
4) If he spaces it wrong to avoid punishment (note that G&W may DELIBERATELY space wrong so that you can't ftilt him), you can toad. If he spaced it right, however, it'll eat the toad.
5) My special trick- I call this shield turnip camping. Pull a turnip, and let G&W bair your shield. AFTER the landing hitbox lands, throw the turnip. Since throwing an item drops your shield, you can use turnips out of shield faster than almost any other attack. This works pretty consistently, and you can sometimes dash attack him too. However, you have to have the turnip pulled in advance.
Since you can, however, use turnips to punish most of G&W's aerials AND ground attacks, camping G&W with turnips and the shield proves surprisingly effective.