npm install postcss-node-sass

postcss([require('postcss-node-sass')]).process(yourCSS);

let gulp = require('gulp'),
    postcss = require('gulp-postcss'),
    sass = require('postcss-node-sass');
gulp.task('css', () => {
    gulp.src('path/to/dev/css')
        .pipe(postcss([
            /* postcss plugins before parsing sass */
            sass()
            /* postcss plugins after parsing sass */
        ]))
        .pipe(gulp.dest('path/to/build/css'));
});
 
/* rest of gulp file */