That only applies to people who don’t own the copyright. If you wrote the original code base, you can license it GPL, but continue to sell it under closed source license as well.
On the other hand, if someone forms it under the GPL, and adds some additional features, the original code base cannot pull those features back in the original closed source project, without ceasing to distribute the whole thing as closed source.
Choosing GPL effectively forms your code, and further code added by others can no longer be added to the “proprietary” branch. But you can continue adding code yourself in parallel in both branches.
> code added by others can no longer be added to the “proprietary” branch
One thing to keep in mind, if you sign over your copyright by agreeing to contribute using a Contributor License Agreement (CLA), the person or organization can take your contributions proprietary.
On the other hand, if someone forms it under the GPL, and adds some additional features, the original code base cannot pull those features back in the original closed source project, without ceasing to distribute the whole thing as closed source.
Choosing GPL effectively forms your code, and further code added by others can no longer be added to the “proprietary” branch. But you can continue adding code yourself in parallel in both branches.