I think this is the crux of the issue. Anyone is of course free to roll their own GitLab (or facebook, or news channel). But this ignores reality.
Facebook control ~90% of social media. There's a very high chance that Facebook could sway every election in America (and lots of other countries also) if they truly wanted to.
I think we are entering a new era. Just as it required a paradigm shift to outlaw anti-competitive practices, I think we need to re-consider what rights these platforms have around speech.