Ruby/GSL provides all the GSL special functions as module functions which are named as the same of the C functions. For example, to compute the Bessel function J0(x), one may invoke the gsl_sf_bessel_J0 function,
require 'gsl' x = 1.0 p gsl_sf_bessel_J0(x) <---- "full" name
Alternatively, including the module GSL_Sf,
require 'gsl' include GSL_Sf p bessel_J0(x) <---- withoug the predix "gsl_sf"
See GSL manual for the complete list of the special functions.