2017 – Edit: anyone trying to set up React in Gulp for a new project: Just use create-react-app

Step I: Add the following to your gulpfile.js somewhere

gulp.task('apply-prod-environment', function() {
    process.env.NODE_ENV = 'production';
});

Step II: Add it to your default task (or whichever task you use to serve/build your app)

// before: 
// gulp.task('default',['browsersync','watch'], function() {});
// after:
   gulp.task('default',['apply-prod-environment', 'browsersync','watch'], function() {});

OPTIONAL: If you want to be ABSOLUTELY CERTAIN that you are in prod mode, you can create the following slightly enhanced task instead of the one in Step I:

gulp.task('apply-prod-environment', function() {
    process.stdout.write("Setting NODE_ENV to 'production'" + "\n");
    process.env.NODE_ENV = 'production';
    if (process.env.NODE_ENV != 'production') {
        throw new Error("Failed to set NODE_ENV to production!!!!");
    } else {
        process.stdout.write("Successfully set NODE_ENV to production" + "\n");
    }
});

Which will throw the following error if NODE_ENV is ever not set to ‘production’

[13:55:24] Starting 'apply-prod-environment'...
[13:55:24] 'apply-prod-environment' errored after 77 μs
[13:55:24] Error: Failed to set NODE_ENV to production!!!!