You just need to create an json report, store it in a Gitlab artifact and mark it as a code quality one:

phpstan:
  image: my_image_with_phpstan:8
  script:
    - phpstan analyse src/ --error-format gitlab --no-progress > phpstan.json
  artifacts:
    when: always
    reports:
      codequality: phpstan.json

Source: https://www.howtogeek.com/devops/how-to-create-a-gitlab-ci-pipeline-to-statically-analyse-php-projects/