But if copyright didn't exist then there would be no purpose for the GPL.

But I want to be able to require that people who redistribute my code also provide the source.

Most of the time, GPL code is stolen without the person who wrote it even being aware. And most of the time they become aware of it, there isn't a whole lot they can do to stop it.

Please define stolen here? I highly doubt that most GPL code is 'stolen' according to the actual terms laid out in the GPL.

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.