Permissive licenses, community, and copyleft

Unfortunately, that isn't a given when you consider "derivative" works. The mere fact that you wrote the code yourself does not guarantee that you have sole control over the copyright.

The issue would be whether the code you wrote for the GPL project might be considered a derivative work of other code in the same project. If so, you might not be able to release it under a more permissive license, because that would amount to relicensing the code is was derived from, and not just the part you wrote.

The problem, as usual, is endemic to copyright: there is no clear, obvious guideline for what constitutes a derivative work, especially in the realm of software.