box: node:4.2.6-slim
dev:
  steps:
    - npm-install
    - internal/watch:
        code: grunt serve
        reload: true

build:

  steps:
    - script:
        code: export NODE_ENV='testing'
    - npm-install
    - script:
        name: echo nodejs information
        code: |
          echo "node version $(node -v) running"
          echo "npm version $(npm -v) running"
    - gulp:
        tasks: build
 
box: node:4.2.6-slim
dev:
  steps:
    - npm-install
    - internal/watch:
        code: grunt serve
        reload: true

build:

  steps:
    - script:
        code: export NODE_ENV='testing'
    - npm-install
    - script:
        name: echo nodejs information
        code: |
          echo "node version $(node -v) running"
          echo "npm version $(npm -v) running"
    - hgen/gulp:
        tasks: build
 
Correct Wercker Gulp build step
Correct Wercker Gulp build step

Apparently there's no first-class Gulp support in Wercker.
https://github.com/hgenru/wercker-gulp-step