I have been looking around for a while and it seems I can't get a straight answer. With all the gcam ports, I have never seen one with sd card support. I know that Pixel doesn't support sd cards, but why can't the support be added to the ports. I have seen a lot of mods that people have added or disabled, but never sd card options.
I don't have a Gcam but, if I can believe the guy in post #7 in this thread- https://forum.xda-developers.com/t/gcam-apk-19-2mp.4028043/ that's the answer to your question. You might also take a look at posts #5 through #9 in this thread- https://forum.xda-developers.com/t/...os-taken-by-google-camera-to-gallery.3752115/. Maybe this- https://forum.xda-developers.com/t/...os-taken-by-google-camera-to-gallery.3752115/ has some applicability.