$ gn gen --help
gn gen: Generate ninja files.

  gn gen [--ide=<ide_name>] <out_dir>

  Generates ninja files from the current tree and puts them in the given
  output directory.

  The output directory can be a source-repo-absolute path name such as:
      //out/foo
  Or it can be a directory relative to the current directory such as:
      out/foo

  --ide=<ide_name>
    Also generate files for an IDE. Currently supported values:
      "eclipse" - Eclipse CDT settings file.
      "vs" - Visual Studio project/solution files.
             (default Visual Studio version: 2015)
      "vs2013" - Visual Studio 2013 project/solution files.
      "vs2015" - Visual Studio 2015 project/solution files.

Should add "xcode" to this list!