gulpfile.js

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

gulp.task('watch:src', function() {
  watch({glob: 'styles/**/*.less'}, function(files) {
    return gulp.start('styles:src');
  });
});

var less = require('gulp-less');
gulp.task('styles:src', function() {
  return gulp.src('styles/**/*.less')
    .pipe(plumber())
    .pipe(less())
    .pipe(plumber.stop())
    .pipe(gulp.dest('dest'));
});