By any chance have you asked Gigabyte Support by opening a ticket with them?
Each Manufacturer might have different size Thermal pads specs due to the GPU cowl size and dimension.
In one Youtube video concerning a RX 580 Thermal pad the Tech removed the old thermal pad and compared the thickness to the new one knowing the old thermal pad may have been compressed somewhat during installation.
Then decided which new thickness thermal pad to use.
You can take the new thermal pad and lay it next to the old thermal pad, while still installed, and see how it compares in thickness.
You can try and purchase a Thermal pad kit with all three sizes.
But I believe asking Gigabyte Support or opening a thread at Gigabyte Forum might be your best bet.
EDIT: Hopefully another User with a Gigabyte RX 470 or 570 might be able to answer your question.
