Another W for Signal

Can only guess, but nixpkgs says the package provided is org.gnome.decibels not decibels. Maybe try this command

It is called pkgs.decibels. Should be feasible if you just did

  environment.gnome.excludePackages = [
    pkgs.decibels
  ];

I thought it was okay with the licensing to sell copies of GPL licensed software as long as the customer had the option to get the source code.