If you want to install a package, search for it at https://search.nixos.org
The gnome system monitor is gnome.gnome-system-monitor for example https://search.nixos.org/packages?channel=23.05&show=gnome.g...
The gnome system monitor is gnome.gnome-system-monitor for example https://search.nixos.org/packages?channel=23.05&show=gnome.g...
And if this doesn't work, go to https://github.com/NixOS/nixpkgs/issues and search for the name (in both open and closed issues and PRs).
It sure beats my previous method of finding packages, which was just blind guessing.
I've updated the post to include the correct package name and how to find it.