Be aware having a GPD Pocket 2 myself the Device is incredibly powerful, but the battery died after 1 year, now it's more or less unusable (crashes on 100% load without battery, even if i have a really strong usb-c charger)
the new one-mix battery seemed to have problems though, and i thought the problem was that i may have gotten after-market batteries instead of an original one.
but now i figured out how to contact the manufacturer and they asked me to send in the device (which i did without sending the battery) and they discovered that the battery controller was broken. they fixed it and added a new battery for a total of 30USD + shipping.
the gpd pocket battery cost a similar amount.
so the problem is really that getting batteries from overseas adds to the cost.