Stock OP camera app is phenomenal already, but I do think GCam is slightly better. Contrast and white balance is a bit better, colors are more true to life, night mode is way better (not that OP has a bad night mode), HDR is better, selfie camera is night and day better, sharpness is pretty much the same. Unfortunately I'm not aware of a way to enable 48mp shots, but honestly the 12mp photos you get are more than good enough.
With a magisk mod made for the 7 Pro, you can enable all 4 cameras to be used on the 8 Pro with GCam.
TL;DR: OP camera is great, but GCam is better. Most probably won't even notice much of a difference.