This work describes the setup of an advanced technical infrastructure for collaborative software development (CDE) in large, distributed projects based on GitLab. We present its customization and extension, additional features and processes like code review, continuous automated testing, DevOps practices, and sustainable life-cycle management including long-term preservation and citable publishing of software releases along with relevant metadata. The environment is currently used for developing the open cardiac simulation software openCARP and an evaluation showcases its capability and utility for collaboration and coordination of sizeable heterogeneous teams. As such, it could be a suitable and sustainable infrastructure solution for a wide range of research software projects.