Where are the .glsl files located in GPD XD? I tried to find them but I only could find .glslp files, which I cannot edit.
Default location is: /data/data/com.retroarch/shaders/shaders/ (you need root to open this directory)
But you can do it another way.
In Retroarch go to - settings/directory/video shader - and point to any othet directory where you want to store your shaders and then in "online updater" download them again, and they will be stored there.