There's a reason GitHub is the de facto host for OSS projects, I'd posit that open source maintainers who deliberately choose an unconventional host like GitLab already increase the barrier to contributions (not just due to network effects, but an unfamiliar UI, feature parity, poor performance etc) and this change would've already tipped the balance in my view and made it even clearer GitLab is not the right choice…
If you believe that it's important for free software to use free software infrastructure (and you should[0]), then there is no choice but to do so. Infrastructure like GitHub is primarily a function of network effects and the only way to overcome them is to cast your lot appropriately. A maintainer who "doesn't care" if their infrastructure is proprietary is not doing a good job. [0]: https://drewdevault.com/2022/03/…
This is why pure FOSS hasn't, doesn't, and won't win. This overly moralistic posturing at every turn drives people away.