The GNU Scientific Library (GSL) is a numerical library for C and C++ programmers. The library provides a wide range of mathematical routines such as random number generators, special functions and least-squares fitting. There are over 1000 functions in total with an extensive test suite.
Version 1.16. Freely available for every user.
The difference between -gcc and -intel version is in used build environment. Intel version should be faster, GCC version is more verified so possibly more stable.
GNU/GPL v3, see http://gplv3.fsf.org/ for details.
Example of module initialization:
module add gsl-1.16-gcc
After initialization are available program binaries and libraries.