Ruby/GSL provides all the GSL special functions (about 400) as modules 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 module function,
require 'gsl' include GSL x = 1.0 p gsl_sf_bessel_J0(x)
For convenience, all the module functions are aliased to names without the prefix "gsl_sf", so the example above is equivalent to a call
p bessel_J0(x)
See GSL manual for the complete list of the special functions.