gistfile1.js

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

var paths = {
  lessFile: 'includes/css/styles.less',
  lessFiles: 'includes/css/*.less',
  cssDir: 'includes/css'
};

gulp.task('less', function () {
  // Compile less into CSS
  return gulp.src(paths.lessFile)
    .pipe(less())
    .on('error', gutil.log)
    .pipe(gulp.dest(paths.cssDir));
});

// Rerun the task when a file changes
gulp.task('watch', function() {
  gulp.watch(paths.lessFiles, ['less','css']);
});

// The default task (called when you run `gulp` from cli)
gulp.task('default', ['watch', 'less']);

More complete example here: https://gist.github.com/andrew-dixon/1d9e820e70b05d800bf0