Within my capacity, help is welcome. As I doesn’t know Java and Maven (I am a C++ addict), I really have no opinion about the right way to do the job in the Orekit context. So, do not hesitate to express your point of view and your needs.
You are right, Gitlab comes with its own CI tool and the latter works fine, in particular with containers. I already use it on a couple of personal projects and some of my colleagues use it on internal projects. But this tool lacks of reporting capabilities. It does not collect, process and display the reports generated by the QA tools like JUnit, Jacoco or Checkstyle. From this point of view, it represents a regression.
So, for the moment, we decided to keep Jenkins but the situation is far from being immutable! 
Sébastien