Download this file: .gitlab-ci.yml and place it in the top-level directory of your repository.
.gitlab-ci.yml filename starts with a dot, this means it is a hidden file (by convention) on Unix.
Commit and push it, and watch the "CI/CD" subsection of your project get populated with "Pipelines" and "Jobs". You can download the artifacts for each job.
By default, these are builds for Linux and Windows.
.gitlab-ci.yml linked above does not rely on our Castle Game Engine Docker image because the disk space on GitLab shared runners seems too small to unpack it. We instead download FPC and build CGE at each run.
But if you use your own runners, disk space may not be a problem. If you want you can change the YAML file to use Docker images (to have more power and build easily e.g. also for Android). Simply uncomment the section in that file marked as
# Commented out version using Docker image.