1. Key Difference 1: Architecture and Scalability: GitLab CI is designed as a native part of GitLab, providing a single integrated platform for version control, issue tracking, and continuous integration. It uses a distributed architecture that allows horizontal scaling by adding more nodes to the system. On the other hand, GoCD is a standalone continuous delivery server that requires separate installation and configuration. It uses a master-agent architecture that can be scaled vertically by adding more resources to the master server but cannot be scaled horizontally like GitLab CI.

2. Key Difference 2: Pipeline Configuration: GitLab CI uses a YAML-based configuration file called .gitlab-ci.yml to define pipelines and jobs. The configuration file is stored along with the project's source code in the repository. GoCD, on the other hand, uses a graphical interface to define pipelines, stages, and tasks. The pipeline configuration is stored outside the project's source code in XML format, making it centralized and separate from the codebase.

3. Key Difference 3: Plugin Ecosystem: GitLab CI has an extensive plugin ecosystem, with a wide range of community-developed plugins available for integration with other tools and services. These plugins can be used to extend the functionality of GitLab CI and integrate with various third-party services. GoCD also supports plugins, but the available options are more limited compared to GitLab CI. It primarily focuses on integrating with popular CI/CD tools and services rather than providing a wide range of plugins.

4. Key Difference 4: User Interface: GitLab CI provides a user-friendly web-based interface that is well-integrated with the GitLab platform. Users can easily navigate through pipelines, jobs, and artifacts, making it simple to visualize and monitor the CI/CD process. GoCD offers a web-based user interface as well, but it is not as user-friendly and intuitive as GitLab CI. The interface requires more technical expertise to configure and manage pipelines.

5. Key Difference 5: Agent Allocation and Load Balancing: GitLab CI allows dynamic and automatic allocation of CI/CD runners based on the project's needs. It also supports load balancing by distributing the workload across multiple runners. GoCD, on the other hand, requires manual configuration of agents and does not offer built-in load balancing functionality. The allocation and management of agents need to be manually handled by the administrators.

6. Key Difference 6: Integration with Version Control Systems: GitLab CI is tightly integrated with GitLab's version control system, providing seamless integration for source code management, issue tracking, and continuous integration. It is optimized for Git workflows and supports Git-specific features like GitLab's code review and merge request system. GoCD, on the other hand, supports integration with multiple version control systems, including Git, Subversion, Mercurial, and more. It provides flexibility for organizations using different version control systems.

In Summary, GitLab CI and GoCD differ in terms of architecture and scalability, pipeline configuration, plugin ecosystem, user interface, agent allocation and load balancing, and integration with version control systems.