I don't understand this. If there were no copyright, people would be able to copy everything they see anywhere and never be forced to disclose their changes like they do with the GPL, which is at least one intention of GPL.
GPL relies on copyright completely, if it didn't exist, I would guess the FSF would come up with some alternative like basing contribution/releases/disclosure/etc around contract law.
The ideal outcome for that point of view is that it would be a direct legal requirement that software is distributed in preferred modifiable form. No licenses or contracts necessary.
GPL relies on copyright completely, if it didn't exist, I would guess the FSF would come up with some alternative like basing contribution/releases/disclosure/etc around contract law.