GitLab's Auto DevOps functionality provides predefined CI/CD configuration that automatically detects, builds, tests, deploys, and monitors your applications. Let's perform the following steps to enable the Auto DevOps option for your existing project:

1. Log in with your project user account.
2. On the Welcome to GitLab screen, you will see links that will help you get started. Here, click the Configure GitLab button to access the configuration options:

3. Only the project users with Maintainers and Admin permissions have access to the project settings. From the Admin Area menu on the left-hand side of the screen, select the Settings | CI/CD menu to access the CI/CD options. The following screenshot shows where the CI/CD settings are located:

4. Under the following Continuous Integration and Deployment page, make sure that the Default to Auto DevOps pipeline for all projects checkbox is checked. Optionally, enter your base domain if you want to use the Auto Review Apps and Auto Deploy features:

5. Click on the Save changes button.