Ive not tested this on GPE firmware but you can give it a go. If there is such a file (mixer_path), and there are the said lines, then try to change it. As usual, perform a nandroid backup in case anything goes wrong. Use this method ONLY if the said files and lines are present okay? I can safely say that it works on CM based roms.
forum.xda-developers.com/moto-g/general/headphones-volume-increase-cm11-kitkat-t2830060 <---- I'm talking about this