Almost there... I can feel it.

jay_get_number

# Line indicated below is generating the error: "get_number() not implemented in class 'FixedFloatArray'"

.sub '' :anon :load
.....
    # Documented http://www.gnu.org/software/gsl/manual/html_node/Mean-and-standard-deviation-and-variance.html if you want to check the signature...
    dlfunc nci, lib, 'gsl_stats_mean', 'dpll'  
    set_global ['GSL'], 'gsl_stats_mean', nci
......
.end

sub mean(*@args) {
    return Q:PIR {
        .local num ans
        .local pmc gsl_stats_mean
        $P0 = new ["FixedFloatArray"], 2
        $P0[0] = 2.71828
        $P0[1] = 3.14159
        gsl_stats_mean = get_global ['GSL'], 'gsl_ran_gaussian_pdf'
        ans = gsl_stats_mean($P0, 1, 2)  #################### <<<<<<<<--------------
        %r = box ans
    };
}