There's already a whole thread explaining about all this, but w/e here goes.
So G&W has a 4 frame jump squat. This is only important because jump squat length determines the DACUS timing. For a character with a 4 frame jump squat you only get 1 frame, that's one-sixtieth-of-a-second to do the DACUS. That one frame window is so tight that you actually CANNOT do it with the Z button. The reason the Z button does not work is because it's a "grab button" and there is actually no single grab input in Smash. When you press Z it inputs shield for one frame and then inputs attack the very next frame. And there maybe you can see the problem. That's two frames for a one frame input, making it literally impossible.
This whole jump squat frame window thing is also the reason why some characters have it easier than others when it comes to inputting the DACUS. In Brawl, Snake had a really lenient window so he could easily DACUS with Z. But there it is still possible to DACUS with that one frame window. You just have press a straight up attack button on the right frame. It's very possible to "claw" your finger over to the A button for the input. But you can also try doing it with a custom control layout like L or R set to attack.
As for the Gatling Combo, the Z button is inputting grab so you are always going to get a grab when you press Z. It's the same scenario you have to use a dedicated attack button to get a real gatling combo. Luckily though Gatling Combo's are lenient enough that you can use the C-Stick for both inputs.
Once you get the hang of it, there is actually a little bit of freedom in how you input these techniques. I personally use C-Right > A to do DACUS, and C-Right > L to do Gatling, because that's what felt the easiest to me.
