My Gulpfile using ES6 (Babel), Browserify, BrowserSync, SASS, Sourcemaps, and more...

gulpfile.babel.js

import critical from 'critical';
import babelify from 'babelify';
import browserSync from 'browser-sync';
import browserify from 'browserify';
import buffer from 'vinyl-buffer';
import gulp from 'gulp';
import plugins from 'gulp-load-plugins';
import source from 'vinyl-source-stream';


/* ----------------- */
/* Development
/* ----------------- */

gulp.task('development', ['scripts', 'styles'], () => {
    browserSync({
        'server': './',
        'snippetOptions': {
            'rule': {
                'match': /<\/body>/i,
                'fn': (snippet) => snippet
            }
        }
    });

    gulp.watch('./client/scss/**/*.scss', ['styles']);
    gulp.watch('./client/scripts/**/*.js', ['scripts']);
    gulp.watch('./*.html', browserSync.reload);
});


/* ----------------- */
/* Scripts
/* ----------------- */

gulp.task('scripts', () => {
    return browserify({
        'entries': ['./client/scripts/main.js'],
        'debug': true,
        'transform': [
            babelify.configure({
                'presets': ['es2015', 'react']
            })
        ]
    })
    .bundle()
    .on('error', function () {
        var args = Array.prototype.slice.call(arguments);

        plugins().notify.onError({
            'title': 'Compile Error',
            'message': '<%= error.message %>'
        }).apply(this, args);

        this.emit('end');
    })
    .pipe(source('bundle.js'))
    .pipe(buffer())
    .pipe(plugins().sourcemaps.init({'loadMaps': true}))
    .pipe(plugins().sourcemaps.write('.'))
    .pipe(gulp.dest('./build/js/'))
    .pipe(browserSync.stream());
});


/* ----------------- */
/* Styles
/* ----------------- */

gulp.task('styles', () => {
    return gulp.src('./client/scss/**/*.scss')
        .pipe(plugins().sourcemaps.init())
        .pipe(plugins().sass().on('error', plugins().sass.logError))
        .pipe(plugins().sourcemaps.write())
        .pipe(gulp.dest('./build/css/'))
        .pipe(browserSync.stream());
});


/* ----------------- */
/* HTML
/* ----------------- */

gulp.task('html', ['cssmin'], () => {
    return gulp.src('index.html')
        .pipe(critical.stream({
            'base': 'build/',
            'inline': true,
            'extract': true,
            'minify': true,
            'css': ['./build/css/style.css']
        }))
        .pipe(gulp.dest('build'));
});


/* ----------------- */
/* Cssmin
/* ----------------- */

gulp.task('cssmin', () => {
    return gulp.src('./client/scss/**/*.scss')
        .pipe(plugins().sass({
            'outputStyle': 'compressed'
        }).on('error', plugins().sass.logError))
        .pipe(gulp.dest('./build/css/'));
});


/* ----------------- */
/* Jsmin
/* ----------------- */

gulp.task('jsmin', () => {
    var envs = plugins().env.set({
        'NODE_ENV': 'production'
    });

    return browserify({
        'entries': ['./client/scripts/main.js'],
        'debug': false,
        'transform': [
            babelify.configure({
                'presets': ['es2015', 'react']
            })
        ]
    })
    .bundle()
    .pipe(source('bundle.js'))
    .pipe(envs)
    .pipe(buffer())
    .pipe(plugins().uglify())
    .pipe(envs.reset)
    .pipe(gulp.dest('./build/js/'));
});

/* ----------------- */
/* Taks
/* ----------------- */

gulp.task('default', ['development']);
gulp.task('deploy', ['html', 'jsmin']);

package.json

{
  ...
  "devDependencies": {
    "babel-core": "^6.7.4",
    "babel-preset-es2015": "^6.9.0",
    "babel-preset-react": "^6.5.0",
    "babel-register": "^6.9.0",
    "babelify": "^7.3.0",
    "browser-sync": "^2.12.10",
    "browserify": "^13.0.1",
    "critical": "^0.7.2",
    "eslint": "^0.18.0",
    "eslint-loader": "^0.9.0",
    "eslint-plugin-react": "^2.0.2",
    "gulp": "^3.9.1",
    "gulp-babel": "^6.1.2",
    "gulp-env": "^0.4.0",
    "gulp-load-plugins": "^1.2.4",
    "gulp-notify": "^2.2.0",
    "gulp-sass": "^2.3.1",
    "gulp-sourcemaps": "^1.6.0",
    "gulp-uglify": "^1.5.3",
    "vinyl-buffer": "^1.0.0",
    "vinyl-source-stream": "^1.1.0"
  }
}

Thanks! Super helpful

Might be even more helpful if you could show us the file structure. ;)