A gulpfile with a 'codegen' task (from command line: gulp codegen) that you can use to code-generate files (in my case Android java files) against a ServiceStack /types/metadata.json endpoint ...

DTO.java.nunjucks

{%- extends "package.java.template.nunjucks" -%}
{%- set templateName = "models/DTO.nunjucks" -%}
{%- set subPackage = ".models" -%}
{%- import 'macros.java.nunjucks' as macros -%}

{% block content %}
import java.math.*;
import java.util.*;
import net.servicestack.client.*;
import com.google.gson.annotations.*;
import com.google.gson.reflect.*;

public class DTO
{
{% for operation in metadata.operations %}
    {# do something with the operation #}
{%- endfor %}

{% for type in metadata.types %}
    {# do something with the type #}
{%- endfor %}
}

gulpfile.js

// Gulp docs at: https://github.com/gulpjs/gulp
// Primary libs used are: 
// - https://github.com/devoptix/gulp-nunjucks-api
// - https://mozilla.github.io/nunjucks/

// Sample directory structure for this gulpfile
// src/      <-- this is where my app is
// scripts/  <-- this is where I put node.js scripts to do codegeneration and other things
// scripts/.gulpfile.js
//         package.json
//        /node_modules/
//        /codegen/input/      <-- this is where i put all my codegen files (each file = 1 codegen .java file (could be for any language you want))
//                /templates/  <-- this is where i put all my codegen templates (i.e.: layouts, macros, includes, used by files in input dir)
//                /output/     <-- you could output your codegenerated files here, OR what I do is output them directory to my src directory as part of my app
//                                 (maintains the same directory structure as inside /input)

var gulp = require('gulp')
    ,glob = require('glob')
    ,path = require('path')
    ,lodash = require('lodash')
    ,data = require('gulp-data')
    ,nunjucksRender = require('gulp-nunjucks-api')
    ,request = require('request')
    ,rename = require('gulp-rename')
    ,source = require('vinyl-source-stream')
    ,jeditor = require('gulp-json-editor')
    ,streamify = require('gulp-streamify')
    ;

/****** MODIFY THIS CODEGEN_DATA, ADD ANYTHING YOU WOULD LIKE TO PASS TO THE TEMPLATES *******/
var codegen_data = {
    // required!!!!
    date: new Date(),
    baseDir: './codegen',
    destDir: '../src/app/src/main/java/org/mycompany/droidapp',
    baseJavaPackage: 'org.mycompany.droidapp',
    metadataUrl: 'https://mycompany.org/api/types/metadata.json',

    // add any data you'd like to pass to the templates below
    hello: 'world',
    exclude_request_types: [
    ],
    exclude_response_types: [
    ],
    exclude_dto_types: [
    ]
};

// create some custom filters, functions, extensions here, to use inside your templates, or load them in using
// a require statement to an external file, however you want ...
// var custom_filters = require('./libs/custom_filters.js');
var custom_filters = {
    'camelCase': function(str) { return lodash.camelCase(str); }
};
var custom_extensions = {};
var custom_functions = {};

var metadata_json_transform = function(json /* this is actually an object (not a string) */) {
  // this is the /types/metadata.json of your api (or whatever file you decide to point to), transform it however you want
  // or build a completely different json file ...
  // whatever you put here will be available in your templates using "metadata.<your object>" as a global
  return json; 
}

/****** do advanced editing as needed below ******/

var nunjucksDate = require('nunjucks-date')
nunjucksDate.setDefaultFormat('MM/DD/YYYY, h:mm:ss a');
custom_filters['date'] = nunjucksDate;

var CODEGEN_INPUT_SRC = codegen_data.baseDir + '/input';
var CODEGEN_DEST_PATH = codegen_data.destDir;
var CODEGEN_TEMPLATES = codegen_data.baseDir + '/templates';

var SS_METADATA_FILE = codegen_data.metadataUrl.substr(codegen_data.metadataUrl.lastIndexOf('/') + 1);
var SS_METADATA_FILE_PATH = CODEGEN_INPUT_SRC + '/' + SS_METADATA_FILE;

gulp.task('codegen_metadata', function () {
    return request({ url: codegen_data.metadataUrl })
        .pipe(source(SS_METADATA_FILE))
        .pipe(streamify(jeditor(metadata_json_transform, {'indent_char': '\t', 'indent_size': 1})))
        .pipe(gulp.dest(CODEGEN_INPUT_SRC));
});

gulp.task('codegen', ['codegen_metadata'], function () {
    return gulp.src(CODEGEN_INPUT_SRC + '/**/*.nunjucks')
        .pipe(data(function() {
            return lodash.merge({}, codegen_data, { metadata: require(SS_METADATA_FILE_PATH) } );
        }))
        .pipe(nunjucksRender({
            src: CODEGEN_TEMPLATES
            ,extension: '.java'
            ,renderString: true
            ,extensions: custom_extensions
            ,filters: custom_filters
            ,functions: custom_functions
        }))
        .pipe(gulp.dest(CODEGEN_DEST_PATH));
});

macros.java.nunjucks

{%- macro header(date, template, apiUrl, metadataUrl) -%}
/*
Code generated on: {{ creation_date | date }}
Template: {{ template }}
Api at: {{ apiUrl }}
Metadata at: {{ metadataUrl }}
*/
{%- endmacro -%}

package.java.template.nunjucks

{%- import 'macros.java.nunjucks' as macros -%}
{#-
     This is a comment block.

     This file is a layout/master page kind of file inherited by child templates in the /input directory ...

     1) Notice the macro below: 'macros.header' which comes from the macros.java.nunjucks file imported above ...
     2) The variables data, metadataUrl, baseJavaPackage come from the codegen_data object in the gulpfile.js
     3) The variable metadata.config.baseUrl is from the metadata.json file from ServiceStack
     4) Other variables like 'subPackage', 'templateName' are assigned in the child templates using 
        {% set subPackage = '.models' %} syntax
-#}
{{ macros.header(date, templateName, metadata.config.baseUrl, metadataUrl) }}

package {{ baseJavaPackage }}{{ subPackage }};

{% block content -%}
{%- endblock -%}