The GDK is not magical. You can take a lead platform and port to the others or design for scaleability and the best way is to stay as close as your minimum common denominator and give you levers and knobs to twist when you scale up.
On mobiles you have the mother of scaleable SDK’s and it does not make this easy.
Who said it was magical? It was said to be made for ease of use for the devs to port their games on PC/Consoles. It's always up to the devs to make an effort.