GPL does not prevent someone from using it as part of something else, so long as that other thing abides to the terms too. In particular, GPL and many open-source licenses require attribution. The fact that Copilot spits out code from other places without attribution clashes with that limitation.

Whether you're allowed to upload GPL code to GitHub or not depends on whatever their Service is at the moment, since the terms say you grant them all the rights "necessary to provide the Service".