This is an excellent Repository with special Install-Images, Drivers and Tweaks for this machine:
git pull ./update.sh kde ./update-kernel.sh
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!