Isn't there any GBA emulator that allows online trading?
mGBA has had an open issue for link cable-over-IP support since 2021. If the conversation there is anything to go off of, endrift's working on implementing some sort of system in which two emulation instances are run in sync, one for each player's inputs, to prevent link cable latency from becoming an issue. The feature's targeted for the 0.12.0 release, but "that may slip depending on how much other stuff I have to do."
That being said, that thread also mentions Multigba S as an alternative that implemented some form of netplay. Apparently it's a fairly lazy implementation that makes use of what's effectively just game streaming, but it gets the job done.
Why do people still use github for this stuff? It just gets DMCA'd. Gitlab seems more reliable.
Eh, Github has its problems, but I wouldn't say that any major source hosting platform is "more reliable" than another in the sense that you're talking about. Github projects might be a bit more visible than Gitlab ones just by nature of the site's reach, but either site is going to comply with any remotely credible takedown notice that Nintendo hands them. The only difference is how long it takes the rightsholders to find the project and take action, not whether or not they will in the first place. The alternative for the hosting platforms would be to risk being held liable for enabling copyright infringement, and no legitimate business's legal counsel is going to endorse that plan.
The only real answer to that problem would be either safeguarding your project from copyright infringement allegations in the first place (see discussions upthread) or finding a site to host your stuff on that is somehow not beholden to international copyright law. The latter would mean leaning on some kind of hypothetical warez distributor that would need to implement easy, anonymous, git-based collaboration. I can't imagine that there's a big enough demand for a service like that to exist, especially since open-source projects generally need to make contributing as accessible as possible in order to progress.
(As a reminder, if you, dear reader, are aware of such a site's existence, keep that to yourself.)
Point being, if you want your project to be copyright-safe, then you need to have a copyright-safe project from the start. Moving from one public, law-abiding hosting platform to another isn't going to save you from an army of lawyers whose literal job is to take your project down.
(Once again, I am not a lawyer and this is not legal advice.)