Thanks @hpy @julianstirling. I believe github.com derives their list from choosealicense.com. See this and this. And it looks like gitlab.com uses the same source for their license templates. See here.
So being accepted by choosealicense.com seems to be a prerequisite to be listed in github.com and gitlab.com. The instructions to be included in the choosealicense.com list seem quite straight-forward. Once you are sure to fulfil all conditions, you initiate a pull request. We currently can easily tick all boxes except the proof for being used in more than 1,000 public repositories. If any of you has the required skills and time to work on that, and also to open the pull request, I would be super grateful. An example for a github search is given in the instructions. I would of course make myself available to discuss in the pull request thread and answer any questions which may arise.
Cheers,
Javier