Are you using flakes? You need to use the legacyPackages output for your system, e.g.

  buildInputs = [ nixpkgs.legacyPackages."x64_64-linux".gcc ];

In practice, it usually looks something like this:

  let
    system = "x86_64-linux";
    pkgs = nixpkgs.legacyPackages.${system};
  in stdenv.mkDerivation {
    ...
    buildInputs = [ pkgs.gcc ];
    ...
  }