A minimal example of configuration with networkmanager and without GNOME

configuration.nix

{ config, pkgs, ... }:

{
  fileSystems."/boot" =
    { device = "/dev/sda1";
      fsType = "vfat";
    };

  fileSystems."/" =
    { device = "/dev/sda2";
      fsType = "btrfs";
    };

  boot.loader.grub = {
    enable = true;
    version = 2;
    device = "/dev/sda";
  };

  networking = {
    hostName = "nixos";
    networkmanager.enable = true;
  };

  nixpkgs.config.packageOverrides = pkgs: {
    networkmanager_openvpn = pkgs.networkmanager_openvpn.override { withGnome = false; };
    networkmanager_pptp = pkgs.networkmanager_pptp.override { withGnome = false; };
    networkmanager_l2tp = pkgs.networkmanager_l2tp.override { withGnome = false; };
    networkmanager_vpnc = pkgs.networkmanager_vpnc.override { withGnome = false; };
    networkmanager_openconnect = pkgs.networkmanager_openconnect.override { withGnome = false; };
  };
}