Quick example of a Gulpfile and the layout I use. In development I'd use gulp watch.

gulpfile.js

var gulp = require('gulp'),
    /* DEPENDENCIES */
    gulp_sass     = require('gulp-sass'),
    gulp_clean    = require('gulp-clean'),
    gulp_source   = require('gulp-sourcemaps'),

    run_sequence  = require('run-sequence'),

    src       = 'src_dir',
    assets    = '/assets',
    css       = '/css',
    data      = '/data',
    html      = '/html',
    js        = '/js',
    lib       = '/lib',
    sass      = '/sass',
    swig      = '/swig',
    ts        = '/ts',

    src_css   = src + css,
    src_js    = src + js,
    src_js_lib= src + js + lib,
    src_html  = src + html,
    src_sass  = src + sass,
    src_swig  = src + swig,
    src_ts    = src + ts,

    trg       = 'target_dir',
    trg_assets= trg + assets;

 
/* STYLES */
gulp.task('sass', function () {
  gulp.src(src_sass + '/main.scss')
      .pipe(gulp_source.init())
      .pipe(gulp_sass({
        errLogToConsole: true  
      }))
      .pipe(gulp_source.write())
      .pipe(gulp.dest(src_css));
});


/* WATCH */
gulp.task('watch:dev', function(){
  /* SASS */
  gulp.watch([src_sass + '/*.scss'], ['sass']);
  /* SWIG */
  gulp.watch([src_swig + '/*.swig'], ['swig']);
  /* DEPLOY */
  gulp.watch([src_css + '/*.css', src_html + '/*.html'], ['deploy']);
});


/* DEPLOYMENT */
gulp.task('deploy', ['clean'], function(){
  /* HTML */
  gulp.src(src_html + '/*.html').pipe(gulp.dest(trg));
  /* CSS */
  gulp.src(src_css + '/main.css').pipe(gulp.dest(trg_assets));
});


/* DEVELOPMENT */
gulp.task('watch', function () {
  run_sequence(['sass', 'swig'], 'deploy', 'watch:dev');
});


/* CLEANING */
gulp.task('clean', function(){
  return gulp.src(trg, {read: false})
             .pipe(gulp_clean({force: true}));
});