Great. if i have a SKY3DS, than don't need GW
You still need a Gateway if you want to use emuNAND, play your eShop Backups, use the Gateway Menu instead of the Sky Button... (I'm sure there's more things)
It's more like if you have a Gateway and a Sky then you don't need a real cart of "The Legend Of Zelda: Ocarina Of Time 3D" to use it.