send variable to gulp

gulpfile.js

var isProduction    = true;

if(gutil.env.production === true) {
  isProduction = true;
}


gulp.task('sass-private', function () {
    return gulp.src('resources/assets/scss/styles-private.scss')
    .pipe(plumber({errorHandler: onError}))
    .pipe(sass({compress: false}).on('error', gutil.log))
    .pipe(autoprefixer({
        browsers: ['last 2 versions'],
        cascade: false
    }))
    .pipe(isProduction ? cssnano() : gutil.noop()) 
    .pipe(rename("private.css"))
    .pipe(gulp.dest('public/css'))        
    .pipe(livereload())
    // create version files
    .on('end', function() {
        gulp.start('version');
    });
});