Depends on module gnu8, that you should load before the gsl module:
module load gnu8
module load gsl/2.6
When loading these modules, the following environment variables are created:
- $GSL_DIR which points to folder gsl/2.6
- $GSL_LIB which points to gsl/2.6/lib
- $GSL_INC which points to gsl/2.6/include