I was messaging back and forth with Wade last night.
According to him 5V chargers are more used with devices that have a single cell battery unlike the Win2 with its 7.6v battery.
PD chargers do have a very small market share but with the iPhone X and Switch using PD chargers he thinks more and more of those chargers and power banks will show up on the market and prices will drop.
After the iPhone X was released various companies in China released chargers that also supported PD charging.
Z8750 supported 5v charging because Intel wanted it used in phones, 7y30 is a normal pc cpu and does not support 5v charging in realtime (relying on machine translation here since he's replying to me in Chinese so I'm not entirely sure what he means by realtime in this context).
He also said back when they were making the Win they were worried since there were no mainstream devices in China that used USB-C cables but eventually the market was flooded with them so he's predicting the same will happen again.
So I guess I got my answer, USB-PD chargers only but you gotta make sure it has 12V.
I've been bugging Wade that GPD needs to do an update on the IGG campaign to clarify this for all the backers.
Gotta say its nice having Wade join the GPD discord group, I can ask him technical questions about my test unit which he'll give technical responses back to and people have had several impromptu technical Q&A's with him covering everything from GPD's experiments on a backlight keyboard, their tests with Chinese clones of the Clickable Switch joysticks & various other things.
This is my prototype Win2 at 7W with stock settings vs my Win.
And this is the Win2 at 15W with the CPU & GPU downvolted to -0.050 using Intel XTU.