> Hey yeah it would be for a closed source app available on the App Store.
Dear Haris Ali, It's great that you are finding GSL useful! The license for GSL is deliberately chosen to encourage the development of free/open-source software and to promote software freedom. In the case of fftw they have slightly different goals, and a different situation, since there is a single entity (MIT) that owns all the source code copyright. We hope that you will find a good way to distribute your program as free/open-source software.