Try streamqueue.
var streamqueue = require('streamqueue');
gulp.task('css', function () {
return streamqueue({ objectMode: true },
gulp.src(['dev/css/reset.css', 'dev/css/style.css', 'dev/css/typography.css', 'dev/css/sizes.css']),
gulp.src(['dev/css/*.scss']).pipe(sass())
)
.pipe(concat('main.css'))
.pipe(minifyCSS())
.pipe(gulp.dest('build/css'))
});
This cheatsheet will help you. PDF is here.