How to implement continuous delivery with our GitLab Monorepo structure?

GitLab CI/CD
2

You can setup rules for jobs which would run the job only if some files are changed.
This way you can have a separate unittest and deploy job for each API you maintain in your repository which is executed only if specific API is changed.

You can setup a hidden job with common parameters and use it to extend actual jobs.

stages:
  - unittest
  - deploy

.unittest_template:
  stage: unittest
  rules:
    - if: $CI_PIPELINE_SOURCE == "push" && $CI_COMMIT_BRANCH != "master"
      changes:
        - $APIDIR/*
    - when: never
  other_common_parameters

.deploy_template:
  stage: deploy
  rules:
    - if: $CI_PIPELINE_SOURCE == "push" && $CI_COMMIT_BRANCH == "master"
      changes:
        - $APIDIR/*
    - when: never
  other_common_parameters

My-Api-1:unit test:
  extends: .unittest_template
  variables:
    APIDIR: 'My-Api-1'

My-Api-1:deploy:
  extends: .deploy_template
  variables:
    APIDIR: 'My-Api-1'