GPL doesn't prevent the copyright holder from also distributing the software under another license if they so choose.
Of course, that would have to be a decision by the copyright holders, which is possibly difficult to provide on GitHub given how easy it makes it to upload other people's work, and you would need the agreement of every contributor unless you do copyright assignment.
And you wouldn't be able to license a project using this service (i.e. with Copilot-generated code) under the GPL. But it's already unclear if you can legally do that today.
