Gitlab isn't just a code repository, it's an entire DevOps platform. I don't use GitLab, but my equivalent (Azure DevOps) gives me things like automated builds on checkin, runs unit tests against those builds, and one-click deployments to my test and production environments.
On my local machine, there's nothing stopping me from using any Git client I want, including the Git CLI..
On my local machine, there's nothing stopping me from using any Git client I want, including the Git CLI..