A self-hosted Gitlab instance would make the most sense, IMO.
Moreover, I wouldn't be surprised if they just opened the code, not the development process. And old, frozen projects with no contributing directions are most likely what we'll get at first.
However, hosting OSS code like this is a public service. It would be nice to see the EU commission competing against GitHub for Open Source hosting, but that's probably a pipe dream for now.
The really promising part is what this will enable going forward:
> The dissemination of software under an open source licence will no longer require a Commission Decision.
> The Commission now allows its software developers to contribute to open source projects with improvements that they developed as part of their work.
So it might become at the team's discretion. The code will probably end up being developed on GitHub or somewhere else, with a mirror of sorts on the EU commission's "repository" website.
Why would you want them to get into that? It doesn't seem like it's something appropriate for a government to get into.