The GPD XD+ does not use the same version of Android. IIRC the GPD XD, which I also have, runs KitKat 4.4. The GPD identifies itself to windows as a Nexus 5.
The paths, according to my file app, are /storage/sdcard0 for the internal SD card, and /storage/sdcard1 for the external SD card.
I would love to have launchbox accessing both card but am aware the GPD XD is a vintage device.
I'm not sure what version of Android the GPD XD= runs :(