I know that GrapheneOS provides another simplified camera app and it make shots another way. Visually, it looks like there is not sharpening or, maybe, some Pixel’s tensor features are not being used there?
So, if we install Gcam on Pixel with Graphene, should it work just same as on stock ROM? And what’s even the point of using another software? Is Gcam less secure or less private itself by any means?
Sounds Like all of the features of Gcam are available. From the GOS website:
Pixel Camera (previously known as Google Camera) can take full advantage of the available cameras and image processing hardware as it can on the stock OS and does not require GSF or sandboxed Google Play on GrapheneOS. Direct TPU and GXP access by Google apps including Pixel Camera is controlled by a toggle added by GrapheneOS and doesn’t provide them with any additional access to data. The toggle exists for attack surface reduction.
As to why GrapheneOS has their own camera app. I believe it comes down to needing some sort of default app (shipping an OS without any system apps wouldn’t make much sense) and wanting to reduce attack surface.
Reference
oh, I should definitely read the documentation myself 
thank you so much for such patience and verbosity!
Since all your posts seem to be grapheneOS related, I will mention that GOS has their own forum https://discuss.grapheneos.org/ which can be very helpful. Just don’t post anything that they disagree with unless you want to be accused of harassment.