They add a lot of stuff on top of GNOME and ship a few extra packages to help with gaming. Not sure how they handle snaps these days because I haven’t checked in a while. It looks nice if you like GNOME, but it just isn’t my cup of tea.
I’m currently running Debian Unstable with KDE on my System76, but I’ve also used Fedora and Arch on it just fine. I don’t have nvidia.