Wait, what? I thought the whole point of GPL is that it is viral, i.e. if you use GPL in a part of your shipped program you must release the whole thing.
From GPLv2:
"These requirements apply to the modified work as a whole. If identifiable sections of that work are not derived from the Program, and can be reasonably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a work based on the Program, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it."
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.
LGPL is not GPL. You are for example allowed to dynamic link from proprietary code to LGPL libraries. This is not the case with GPL shared libraries. You should take look at the LGPL license agreement and then see how the code is used.
From GPLv2:
"These requirements apply to the modified work as a whole. If identifiable sections of that work are not derived from the Program, and can be reasonably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a work based on the Program, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it."