is the Gcam app noticeably better than a stock camera app?
Yes, there’s a very noticable difference.
What sorts of things would it do better?
It uses the custom ML chip in the Google Tensor processor for post processing. This makes the photos and videos look amazing.
Low lighting or blur reduction?
Both, and a lot more.
In my other comment, I outlined a solution for easily installing the Google Camera app.
Linux phones like the Librem 5 are fundamentally insecure. It’s also outdated and overpriced, I really wouldn’t recommend it.