The ALPS sticks GPD use on the GPD WIN (and I think on the XD) are low profile sticks. They don't have center click because of this. To use center click sticks there are two options
1) They can use the ALPS center click sticks , but they are a much deeper part. Therefore the WIN would need to be made deeper ie thicker, maybe 5-7mm thicker.
2) They can use a low profile clickable nub, like those planned for the Pyra.
Mind you, if they do make it thicker, it would also open the door for a bigger battery. Or perhaps even a case that included a battery door for battery quick change over.