Currently Behave Pro Cloud, Server and Data Center doesn't support connecting to git code repositories hosted in GitLab.com or GitLab on-premise. We do plan to add GitLab support in the near future but we are taking a cautious approach to building the solution.
Both GitHub and Bitbucket have a more mature integration model that supports Apps. Administrators can install apps into select repositories and assign permissions to the App.
GitLab doesn't have the concept of Apps would have to be install as an individual OAuth user (usually an administrator) and this means Behave Pro could have permissions beyond what a typical Jira users would have. We use the App model with our GitHub and Bitbucket integration to avoid these issues around security privilege escalation.
We are building GitLab support but we want to make sure proper security measure are in place and this requires us to take more time.