To start creating an automatization between Gitlab and Pantheon we need to following some steps.

1. Create a file and script to make the magic and pushing everything to Pantheon from Gitlab.

   Basic Script:

   stages:  
   - deploy before_script:  
   # See https://docs.gitlab.com/ee/ci/ssh_keys/README.html  
   - eval $(ssh-agent -s)  
   - echo "$SSH_PRIVATE_KEY" | tr -d '\r' | ssh-add - > /dev/null  - mkdir -p $HOME/.ssh && echo "StrictHostKeyChecking no" >> "$HOME/.ssh/config"  
   - git config --global user.email "$GITLAB_USER_EMAIL"  
   - git config --global user.name "Gitlab CI"
   deploy:dev:  
   stage: deploy  
   environment:    
   name: dev    
   url: https://dev-$PANTHEON_SITE.pantheonsite.io/  
   script:    
   - git config checkout.defaultRemote origin    
   - git remote add pantheon $PANTHEON_GIT_URL    
   - git fetch pantheon    
   - git branch -v    
   - git checkout master    
   - git push pantheon master --force  
   only:    
   - master

   Gitlab by default use ".gitlab-ci.yml" as a name for the file

2. Get this code and create a new file with the name .gitlab-ci.yml in your project root.
3. Commit and push this file to the repository project in Gitlab on the master branch.
4. Once you have this file in the Gitlab repository, let's configure some variables used on the script. For example $PANTHEON_GIT_URL and $PANTHEON_SITE
5. To configure these variables we need to go to Gitlab Project page then in the left sidebar menu there is the a Settings option. Click on this menu item and then click on CI/CD submenu item. 
6. Then go to that page and go to the Variables section
7. Now create the variables that you used on the script. In this case we used $PANTHEON_GIT_URL and $PANTHEON_SITE so now we need to set a value for them.
8. For $PANTHEON_GIT_URL. Go to Pantheon and copy the git url and page on value field. 
9. For $PANTHEON_SITE it is the name of the site. 
10. Click on update variable and then we can start to pushing things automatically on pantheon.
11. Check in the left sidebar the CI/CD menu item all deployments and check the jobs.