Did you miss our announcement at DrupalCon Pittsburgh? GitLab CI is now available for every project on Drupal.org. If you maintain a project you can now configure GitLab CI for all of your testing needs. 

The Drupal Association and community contributors have created a default testing template that will help you get started, and should be automatically kept up to date with new versions of Drupal Core and its system requirements. But if you're a CI power user, you can customize the testing in any way you please.
In the short term, DrupalCI is still available so you can use the systems in parallel to make sure your GitLabCI setup is working for your project. But once it is working, we ask you to disable DrupalCI testing on your project to save testing resources. 
Please read the documentation for getting started with GitLab CI.
The Drupal Association spends more than $10,000/month on testing infrastructure alone. If you would like to support our work you can donate, become a member, or become a partner.