This is an excellent Repository with special Install-Images, Drivers and Tweaks for this machine:

https://github.com/stockmind/gpd-pocket-ubuntu-respin/

Petr Matula fixed it :) Thx!

https://www.reddit.com/r/GPDPocket/comments/76svuy/linux_headphones_auto_switch_fix/

https://bitbucket.org/simone_nunzi/gpdpocket-kernel/downloads/gpdpocket-headphone-auto-switch.zip

Run install script and reboot/relaunch acpid daemon, then try to plug headphones and check if it's working

For me it works!