Im having pretty good luck throwing bog standard gnu/linux distros on mine as long as the kernel is recentish. About the only thing I need to do is rotating the display and touch screen in XOrg.
GPD hardware is seemingly remarkably standard. Which I approve of.
Though I think that Ubuntu MATE package is nearly official at this point. GPD has been offering Ubuntu OEM style with their last few models and I suspect its in partnership with those folks.
Id still recommend checking out compatability reports after new models come out. ( They had a recent accident with linux support and the MicroPC model I think. ) But GPD has been doing a good job.
EDIT
Picked up a GPD Pocket 2 off ebay. I am writing very post on it. Had been looking for something lighter weight I could use to tote between home, work and class and the full blown thinkpad is a hair cumbersome. Had considered waiting for the PineBook Pro but I suspect my courses work will demand Windows in a VirtualBox instance at somepoint, which does force me to go x86; and attempts to adapt a chromebook to my needs did not go well.