I have a code project which I have released under the GPL3+, which you can see here. I would like to implement a section of code that would use the lmfit nonlinear fitting library, provided here. The lmfit library is licensed under the 2-clause BSD license, which appears to be compatible with the GPL3+. My question is how to bundle them together. I would like to include the lmfit source code inside my git repo. What is the proper way to handle the licensing? Do I simple leave the BSD license in the lmfit files that I use and leave the GPL clause in my own source code? Does the presence of the BSD license in the repo affect files that I have licensed under the GPL?
EDIT: on further reading, it seems that I can release FreeBSD software under the GPL. SO my question is about how to actually do this properly. Do I include both licenses in the source code? Obviously I have to keep the copyright and BSD licenses intact in those sections of the code, but do I also add the GPL so that the source for lmfit in my git repo would now have two licenses inside it? Do I add my own copyright like I have in my other source code (I am guessing no).