nix-shell -p will cause the package to be downloaded to Nix store and enter a new command line shell where the package is available. It will not install it to your session so the GNOME application launcher will not be aware of it, and if you exit the shell (e.g. by running exit command, pressing Ctrl-D, or closing the Console window), the package will not be available any longer (unless you find the correct path in /nix/store directory).
If you wish to install a package semi-permanently, you can use nix-env command, or more nicely install it declaratively by adding it to environment.systemPackages in your configuration.nix file and rebuilding the system. You can find the installation instructions also on NixOS Search
If you want a graphical software center, there is Nix Software Center: gtk4/libadwaita app store for nixpkgs