If you your going to make these kinds of accusations (the kind that if proved true would lead to multi-million dollar lawsuits), you should at least try to provide sources.
That caveat is irrelevant as it has nothing to do with a private repository. When code is public, especially with a GPL license, it can end up in multiple repositories which may not all de-check the share check.
I tried, all I could find was one other HN comment asking about it. Admittedly I could have tried harder but it really isn't my assertion to defend.
I think there might be some confusion here between private repositories and public repositories with restrictive licenses. There is evidence of the latter but not the former.
Do you have some evidence that github trained copilot on private repositories? They've been pretty clear about claiming they only used public repos.
Also, gitlab is not owned by Google AFAICT but is instead a publicly traded company.