Practicing platzigram in Javascript

I'm doing the Platzigram project of Platzi_ to practice Javascript and so on. When using the package.json to automate tasks with <script> it does not work for me when I execute the npm start command and the gulp watch and tasks are executed > node server.js , only one of them is executed and the one that is first. I will leave the package.json and the gulpfile to tell me if something is wrong and clarify that I am running it in Windows .

package.json :

{
  "name": "photogram",
  "version": "0.1.0",
  "description": "Ver y compartir fotos",
  "main": "server.js",
  "keywords": [
    "photogram",
    "javascript",
    "fotos"
  ],
  "author": "Christian Carballo",
  "license": "ISC",
  "dependencies": {
    "express": "^4.15.2",
    "materialize-css": "^0.98.2",
    "page": "^1.7.1",
    "pug": "^2.0.0-rc.1"
  },
  "devDependencies": {
    "babelify": "^7.3.0",
    "browserify": "^14.3.0",
    "gulp": "^3.9.1",
    "gulp-rename": "^1.2.2",
    "gulp-sass": "^3.1.0",
    "vinyl-source-stream": "^1.1.0",
    "watchify": "^3.9.0"
  },
  "scripts":{
    "build": "gulp",
    "start":"gulp watch & node server.js"
  }
}

The gulpfile :

var gulp= require("gulp");
var sass=require("gulp-sass");
var rename=require("gulp-rename");
var babel= require("babelify");
var browserify=require("browserify");
var source=require("vinyl-source-stream");
var watchify= require("watchify")

gulp.task("styles", function(){
    gulp
        .src("index.scss")
        .pipe(sass())
        .pipe(rename("app.css"))
        .pipe(gulp.dest("public"));
})

gulp.task("assets",function(){
    gulp
        .src("assets/*")
        .pipe(gulp.dest("public"));
})

function compile(watch){
    var bundle=watchify(browserify("./src/index.js"));

    function rebundle() {
        bundle
            .transform(babel)
            .bundle()
            .pipe(source("index.js"))
            .pipe(rename("app.js"))
            .pipe(gulp.dest("public"));

    }

    if (watch) {
        bundle.on("update", function(){
            console.log("-->Bundling...");
            rebundle();
        });
    }

    rebundle();
}

gulp.task("build", function(){
    return compile()

});

gulp.task("watch", function(){ return compile(true); });
gulp.task("default", ["styles", "assets","build"]);