The strict (GPL) vs. permissive (LGPL, MIT,BSD...) open-source licenses is an old debate. They basically correspond to two different visions of what freedom is ("freedom must be protected" vs. "freedom is freedom no matter what", sort of, I'm resuming a LOT). There are arguments on both sides, but they really are both sides of freedom, at the end they all aim at the same thing.
In any case, FreeCAD has a permissive license, and to avoid any further headache (we had a lot already in the past, opensource licenses can be a hell
), we only allow code in FreeCAD with a permissive license.
Personally I'm comfortable with LGPL, I don't care much if part of our code is reused by commercial software, FreeCAD is strong enough to "defeat" attempts to fork it commercially (there have been several in the past), its strength is the community-based development, and hardly a private company would be able to copy that.
That said, you can still decide to license your work under GPL. Only, it cannot be included in the FreeCAD source code. But it can still exist as an addon or macro. The important thing is, FreeCAD cannot depend on strict-licensed code. But strict-licensed code can depend on FreeCAD.