External Pipelines order

In my config file, I have an external pipeline for gulp. I noticed that gulp run before middleman. How can I have gulp run after middleman builds?

Could you please paste your config.rb (only the external pipeline configuration should be OK) ?

# Build-specific configuration
# https://middlemanapp.com/advanced/configuration/#environment-specific-settings

configure :build do
  activate :minify_css
  #activate :minify_javascript
  activate :imageoptim

activate :external_pipeline,
  :name => 'gulp',
  :command => (build? ? 'gulp prod': 'gulp'),
  :source => 'build',
  :latency => 1