Gulp less watch

gulpfile.js

var gulp = require('gulp');
var less = require('gulp-less');

gulp.task('less', function() {
  return gulp.src('css/app.less')
    .pipe(less())
    .pipe(gulp.dest('dist'));
});

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