Using the Gulp-useref plugin
It will concatenate the files that are inside each commented build block. At this link you can see the more detailed documentation.
https://github.com/jonkemp/gulp-useref
<html>
<head>
<!-- build:css css/style.css -->
<link href="bower_components/bootstrap.css" rel="stylesheet">
<link href="css/main.css" rel="stylesheet">
<!-- endbuild -->
</head>
<body>
<!-- build:js scripts/bundle.js -->
<script type="text/javascript" src="bower_components/jquery/dist/jquery.min.js"></script>
<script type="text/javascript" src="scripts/main.js"></script>
<!-- endbuild -->
</body>
</html>
In the case of SASS you must first compile Sass and then include the generated css on the page within the build block. And so generate a file with all the desired css’s.
This plugin performs this task:
https://www.npmjs.com/package/gulp-sass