body {
  color: red;
  font-size: {
    0: 1em;
    $tablet: 1.5em; /* use it with variables */
    1600: 2em;
  }
}

body {
  color: red;
  font-size: 1em
}
@media screen and (min-width: $tablet) { /* use it with variables */
  body {
    font-size: 1.5em;
  }
}
@media screen and (min-width: 1600px) {
  body {
    font-size: 2em;
  }
}

npm i postcss postcss-responsive-properties --save

var postcss = require("postcss"),
    responsiveCSSProperties = require("postcss-responsive-properties");
postcss([ responsiveCSSProperties ])
    .process(css, { from: "src/input.css", to: "dist/output.css" })
    .then(function (result) {
        fs.writeFileSync("dist/output.css", result.css);
        if ( result.map ) fs.writeFileSync("dist/output.css.map", result.map);
});

npm i gulp gulp-postcss postcss-responsive-properties --save

var gulp = require("gulp"),
    postcss = require("gulp-postcss"),
    responsiveCSSProperties = require("postcss-responsive-properties");
gulp.task("styles", function() {
    var processors = [ responsiveCSSProperties ];
    return gulp.src("src/input.css")
        .pipe(postcss(processors).on("error", function(err) { console.log(err)}))
        .pipe(gulp.dest("dist/output.css"));
});