It looks like there is no function inside the gvr library where you can get the real distortion parameters from - only radial distortion parameters, but as I recently found out
https://stackoverflow.com/questions/58303389/daydream-gvr-headset-distortion-model
gvr doesn't use them anymore / also uses tangential distortion parameters.
This makes it (almost) impossible to use a custom undistortion method ( e.g. VDDC )