Before starting gulp we need to install node.js and npm.
Then install gulp-sacc
$ npm i gulp-sass --save-dev // i = install
Gulp Head
var gulp = require('gulp');
// Requires the gulp-sass plugin
var sass = require('gulp-sass');
Gulp Body
gulp.task('sass', function(){
  return gulp.src('app/scss/*.scss') // searching for sass files
    .pipe(sass()) // Converts Sass to CSS with gulp-sass
    .pipe(gulp.dest('app/css')) // destination of the css file
});
Gulp watch
gulp.task('watch', function(){
  gulp.watch('app/scss/**/*.scss', ['sass']); 
  // Other watchers
})