Error trying to get Gitlab CI to run NPM and Gulp

NPM installs correctly, but then I keep getting this error on my deploy.

npm info ok 
$ gulp
/bin/bash: line 48: gulp: command not found
ERROR: Job failed: exit code 1

My .gitlab-ci.yml looks like this

image: node:latest

cache:
  paths:
  - node_modules/

before_script:
  - npm install

pages:
  stage: deploy
  script:
  - gulp
  artifacts:
    paths:
    - public

My package.json includes the following

"scripts": {
  "install": "gulp",
  "test": "gulp test"
},
"devDependencies": {
  "gulp": "^3.9.1"
}

and I know my gulpfile.js is fine as it works great locally, running everything I need on either npm install or simply gulp. Really not sure what I am doing wrong.

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