Not a lawyer and it has been a while since I studied this, but when one open source project uses another, they aren't really transforming the others code into a new license.
When GNU/FSF says a license is compatible with the GPL, they mean you can legally use the code with the GPL. More or less, the FSF says if you use a GPL code the entire project has to give end users all the freedoms in the GPL. The LGPL is slightly different in that it can be a separate library. They consider even dynamic linking a GPL project to require both projects to be covered under GPL.
This is why proprietary developers call the GPL "viral." GPL code "infects" all other code with its license. This is the deal you make when you use GPL code, and I think it is a fair one. You don't have to use their code.
I suggest you read the licensing bits of the Free Software Foundation's website. fsf.org and gnu.org