Following the documentation here https://docs.gitlab.com/ee/user/project/highlighting.html
I added a line to .gitattributes to highlight icpp files as c++.
I don't see the syntax highlighting applied yet, but I seems to be just that only the .gitattributes in the master branch count.
E.g a test in my Gaudi for here shows a highlighted icpp file.
I propose to check this here before I create MRs for other projects.