I use on of two version.

BASEURL is the extra path also defined in _config.yml. In gitlab pages it is normally the project name.

Deploy w/o linkchecking

In .gitlab-ci.yml, test and deploy looks like this

pages:
  stage: deploy
  before_script:
  - bundle install
  script:
  - bundle exec jekyll build -d ./public
  artifacts:
    paths:
    - public
  only:
  - master

Deploy w. linkchecking

In .gitlab-ci.yml, test and deploy looks like this

variables:
  BASEURL: myproject

test:
  stage: test
  before_script:
  - bundle install
  script:
  - bundle exec jekyll build -d ./test/$BASEURL
  - bundle exec htmlproofer --assume-extension --url-ignore '#' --log-level :debug ./test
  artifacts:
    paths:
    - test

pages:
  stage: deploy
  script:
  - cp ./test/$BASEURL public
  artifacts:
    paths:
    - public
  only:
  - master

The snippet deploy pages after test links. See linkchecker for more info.