That’s actually fine (kind of the idea of specifying a license). What is not fine is using that code in non-GPL licensed code.
Actually yes. I'm not against the tech. I'm against using my code without consent for a tool which allows to breach the license I put my code under.
IOW, if Copilot understood code licenses and prevented intermixing incompatibly licensed code while emitting results for my repository, I might have slightly different stance on the issue.