Eh, the vast majority of people running these projects don’t care where it’s hosted.

The reason it’s on GitLab is because it was a free and easy place to put code in the first place.

Mirroring was extremely common when the internet was slower.

There is absolutely nothing unethical here in relationship to the project owners themselves.

This possibly might be damaging to GitHub themselves but even that is a stretch.

I would be extremely surprised if you’ll find project owners that are “offended” by this for lack of a better term and have published their code explicitly on GitHub in order to support GitHub.