1. Integration with GitLab: One major difference between GitLab CI and Semaphore is their integration with GitLab. GitLab CI is an integral part of GitLab, offering seamless integration with its source code management (SCM) features. On the other hand, Semaphore is a standalone CI tool that can integrate with various SCM platforms, including GitLab. This means that if your development workflow is centered around GitLab, GitLab CI provides a more tightly integrated experience.

2. Configuration Syntax: GitLab CI and Semaphore also differ in terms of their configuration syntax. GitLab CI uses a YAML-based configuration file (.gitlab-ci.yml) where users define their pipeline stages, jobs, and associated commands. Semaphore, on the other hand, uses a declarative approach with its own configuration language, which allows for easy definition of build steps, commands, and environments.

3. Runner Infrastructure: Another difference lies in their runner infrastructure. GitLab CI allows the flexibility of using either shared runners (which are provided by GitLab) or dedicated runners (installed on your own infrastructure). Semaphore, in contrast, uses its own cloud infrastructure for runners, eliminating the hassle of managing and scaling runners yourself.

4. Pre-installed Software: GitLab CI and Semaphore also vary in terms of pre-installed software. GitLab CI provides a range of commonly used tools and packages pre-installed on its runners, enabling easier setup and compatibility with various build processes. Semaphore, on the other hand, requires explicit installation of any necessary tools or dependencies, giving users more control over their environment.

5. Pricing Model: When it comes to pricing, GitLab CI and Semaphore have different models. GitLab CI is free to use for public repositories and offers paid plans for private repositories, with pricing based on the number of users. Semaphore, on the other hand, follows a subscription-based model where pricing is based on the concurrent jobs running per month, providing more flexibility depending on your needs.

6. Extensibility and Community Support: A notable difference between GitLab CI and Semaphore is their extensibility and community support. GitLab CI benefits from the larger GitLab community, which offers extensive documentation, resources, and a wide range of community-contributed integrations. Semaphore, although having a smaller community, provides a streamlined and user-friendly user interface with good documentation and support.

In Summary, GitLab CI offers tighter integration with GitLab, uses a YAML-based configuration file, allows for flexibility with runner infrastructure, provides pre-installed software on runners, follows a user-based pricing model, and benefits from the larger GitLab community. Semaphore, on the other hand, can integrate with various SCM platforms, uses a declarative configuration approach, utilizes its own runner infrastructure, requires explicit installation of software, follows a job-based pricing model, and provides a streamlined user experience with good documentation and support.