The decision to move to GitLab was made after a trial period where the Linaro LAVA Software Team tested out a new workflow based around using Docker to run CI on LAVA itself. This new workflow is described in the Contribution Guide
Linaro aims to lead collaboration across the ARM ecosystem using open source software. Linaro does this by promoting contribution of changes upstream and the use of open source software across the ecosystem.
Debian contains a large repository of free and open source software and project members have a lot of experience of writing, packaging and optimising free software. Debian has no corporate sponsor, so maintainers contribute in their own time whilst in employment for other roles. Some maintainers are employed, by various companies, to work on Debian or software available within Debian but the employers have no direct say in how that work is performed within Debian.
Linaro has historically recruited engineers who are also Debian developers and these developers will bring their experience within Debian to how Linaro develops some solutions. There is no formal link between Linaro and Debian, although both seek to promote collaboration through the use of free and open source software.