A public forum (which GitLab isn't - it's a place for code more than politics) that doesn't allow civil discussion of on-topic ideas is arguably a bad thing.
From On Liberty, by JS Mill:
> If all mankind minus one, were of one opinion, and only one person were of the contrary opinion, mankind would be no more justified in silencing that one person, than he, if he had the power, would be justified in silencing mankind. Were an opinion a personal possession of no value except to the owner; if to be obstructed in the enjoyment of it were simply a private injury, it would make some difference whether the injury was inflicted only on a few persons or on many. But the peculiar evil of silencing the expression of an opinion is, that it is robbing the human race; posterity as well as the existing generation; those who dissent from the opinion, still more than those who hold it. If the opinion is right, they are deprived of the opportunity of exchanging error for truth: if wrong, they lose, what is almost as great a benefit, the clearer perception and livelier impression of truth, produced by its collision with error.
Of course, Mill assumed that most people are capable of rational thought, and would generally move towards a more correct view of the world when exposed to a marketplace of ideas. The internet doesn't always work like that. However, the alternative to "free speech" is echo chambers, which is probably worse.
But I don't see why GitLab has any real responsibility to allow stuff that's not code (or is just joke code) - it's not a really a marketplace of ideas, it's a place to share code. And given GitLab has a FOSS version, yes, people can get their own server.