My usual gulp file for compiling sass.

gulpfile.js

// Install command:
// npm i -D gulp gulp-sass gulp-autoprefixer gulp-cssbeautify gulp-cssnano gulp-rename
//
// Replace XXXXXXX with your project name

var gulp = require('gulp');
var sass = require('gulp-sass');
var auto = require('gulp-autoprefixer');
var beau = require('gulp-cssbeautify');
var nano = require('gulp-cssnano');
var name = require('gulp-rename');

gulp.task('sass', function() {
  gulp.src('./sass/XXXXXXX.scss')
    .pipe(sass().on('error', sass.logError))
    .pipe(auto())
    .pipe(beau())
    .pipe(gulp.dest('./'))
    .pipe(nano())
    .pipe(name('XXXXXXX.min.css'))
    .pipe(gulp.dest('./'));
});

gulp.task('watch', ['sass'], function() {
  gulp.watch('./sass/**/*.scss', ['sass']);
});

gulp.task('default', ['watch']);